Battlefleet Gothic is probably the most successful and, unusually, one of the most recently released of Games Workshop's 'Specialist Games'. The background material is set firmly in the milieu of GW's Warhammer 40,000, representing spaceship combat among the various races/armies that make up the world of 40K. The specific background story, in fact, represents the 12th Black Crusade of the Chaos Space Marines led by Abaddon, when they used sorcery to create warp storms to close off the Gothic Sector of Imperial space and attempted to conquer the sector with the help of massive unique ships like the appropriately-named 'Planet Killer'. But, enough of that...
Unlike most miniatures games, BFG is not nearly as costly or time consuming when it comes to assembly and painting of the models. A 1000- to 1500-point fleet for either of the basic races (Imperium and Chaos) can easily be had for under $100 and BFG is one of the few minis games where it actually comes in handy to purchase the basic game box, as you receive the main rulebook in hardcopy (now also available on the web), 4 Chaos cruisers, 4 Imperial cruisers, and a few sheets of cardboard counters (Blast markers, Asteroid areas, Planets, Moons, ship direction markers, attack craft, torpedo salvoes) for only $50. Escorts are typically a single, metal piece, occasionally two, while cruisers are most often two plastic pieces for the hull and 4-6 plastic pieces for weapon batteries, dorsal weapons, bridge towers, and optional antennae. Battleships are typically multi-piece metal models. All come with the well-known GW 'flying' stands.
BFG is a 2-D adaptation of 3-D space, so while ships can easily fire past each other and interspersed enemy ships, it also includes such hoary 19th-century tactics as ramming and boarding. The basic capital ship for every race is the cruiser, typically coming in at 8 hull (or hit) points. Light cruisers are available for some races and tend to be faster but with less structure and shields. Battleships tend to be closer to 12 hull points with greater and longer-range weaponry, while Escorts are almost universally 1 hull point with typically short-ranged and less effective weaponry, but which also commonly fly and fire as a squadron of up to 5.
Strangely enough, BFG movement and range is also calculated in centimeters, unlike the major games of GW's portfolio, so be prepared to either use the provided range sticks in the basic box or to find a dual gauge tape measure; an increasingly rare bird in our ever more backward United States, I've found.
Game function is done in phases in which each player executes all phases for his ships before allowing the opposing player to do the same. The three phases are movement, shooting, and ordnance. The minor exception is that both players' ordnance moves and attacks targets in range in BOTH ordnance phases.
All ships have a Leadership rating rolled at the beginning of each game which indicates how well they're able to take on Special Orders, which allow the ship to bend or break the rules. Passing an Ld test (average Ld is 7 for the two basic fleets) allows the ship to perform actions like All Ahead Full, adding extra movement; Lock On, re-rolling misses when firing; Come to New Heading, which allows capital ships to make extra turns; and the all-important Brace for Impact, which allows ships to make saving throws against attacks. All Special Orders come with one or more restrictions (halving weapon strength, disallowing turns) when used and any SO rolls that are failed end the possibility of making further rolls for that turn. Most races have commanders who can spend part of their allotted points on one use only re-rolls to get around this problem, but it's still a good idea to start rolling for SOs on your high Ld craft and descend from there. Special dice are available from GW to place by your ships with the appropriate symbol to remind everyone what SO they are on.
Movement is an intrinsically tactical element of the game, as most capital ships can only make one 45 degree turn after moving a minimum distance and they must move that minimum distance every turn unless executing a Burn Retros special order which allows them to remain still. Careful planning of your own movement and anticipation of enemy movement is important, as you can easily find yourself with an enemy target out of your firing arc if you do not plan ahead. Movement also frequently determines what heading your opponent's ships will be on relative to your own, which has an impact on the most common guns of most ships. Escorts, unlike capital ships, do not need to move a minimum distance before turning and can usually turn up to 90 degrees but must still move their minimum each turn, unless Burning Retros. As ships get larger, they also often get slower, meaning that Escorts will typically be the fastest, then Light Cruisers, Cruisers, Grand or Battlecruisers, and Battleships. Most Battleships can't even use the Come to New Heading order, as they're simply too ponderous.
The primary weapons are referred to as 'weapon batteries', tremendous plasma and laser arrays that fire over vast distances so that, even though ships are easily visible to one another on the table and eventually even end up in potential ramming and boarding situations, the batteries are supposed to be capable of firing 'over the horizon', as it were, without the gunners being able to physically see their targets. This 'firing solution' simulation (also including the tremendous calculations needed to hit a moving target in 3-D space while one is also moving) is handled in a couple of manners. Firstly, the heading of the ship is important. A target closing on the firing ship is easier to hit, as at least one vector is already pre-determined. A target moving abeam or away from the shooter is, consequently, more difficult. The weapons battery table takes this into account by reducing the number of d6 that can be rolled based on the target's heading (closing, moving away, abeam in descending order) and based on the target being fired upon; capital ships being much easier to hit than escorts or ordnance (salvoes of torpedoes and/or squadrons of attack craft.) There are also negative modifiers for long range that some races are able to ignore and positive modifiers for short range that all can take advantage of.
When the number of dice for each battery is determined those dice are rolled and the results compared to the target's armor value. If that value is equalled or exceeded, hits are scored. If the target has shields, as many hits as the target has shields can be ignored other than producing a blast marker. Once the shields are down, hits deduct from the hull points of the target and dice are rolled for each hit to determine if a critical hit has been done. Critical hits include results like knocking out weapons on various sides of the ship, engine room damage that eliminates turns or lowers speed, and so on. Guns of all kinds (not just 'weapon batteries') can be found in 4 areas on most ships: port, starboard, prow, and dorsal. All weapons will have a firing arc and a maximum range in the stat line of the ship. The direction finder can be used to determine whether a target is in the appropriate arc and can be fired upon and it is legal (unlike in 40K) to measure range to a target before choosing to fire. Weapons on the same ship can fire at all available targets in whatever arc is appropriate and can fire at targets beyond the nearest ones if the ship passes an Ld roll. A favored tactic is splitting the enemy fleet so that weapons on both sides can open up full force (and hopefully gain a weapon battery bonus for being at close range) even if the targets will be abeam.
The other main type of gun is known as the lance battery. Lances are high-energy lasers that have a better chance of scoring a hit than most weapon batteries by ignoring the armor value and always needing a 4 or better. Consequently, lances tend to be fewer in number.
Ordnance is comprised of torpedoes, bombers, fighters, and assault boats. All ordnance ignores a target's shields, but is subject to being shot down by the target's turrets, if any. Torpedoes travel in a straight line and the salvo reduces in number for any that are shot down or that actually cause a hit and then continue in that straight line toward any target beyond the original. For example, a Chaos Repulsive fires 6 torpedoes at a Lunar. The Lunar's turrets shoot down one, so five attacks are rolled against the Lunar's armor value. 2 hit and eliminate two hit points and 2 rolls are determined to see if any critical damage was done. The other 3 torpedoes carry on past the Lunar to impact with any target that they intersect, including friendly ones.
Bombers conduct attack runs against targets and use a quick formula determined by the turret count on the target to determine how many attacks they can do. Fighters cannot damage enemy ships but can be used to wipe out any other ordnance (including torpedoes) that they encounter. Combining flights of fighters and bombers can also increase the number of attacks the bombers make as the fighters absorb the flak from the turrets.
Assault boats land marines onto the enemy craft and automatically roll for a low level critical hit for each attack that gets through, limited to loss of weapon bays up to starting a fire on the ship that must be extinguished before it does more damage.
Blast markers are created by hits on a ship's shields and the destruction of ships. These are clouds of radiation, debris, and so forth that slow the movement of ships flying through them, potentially damage ships that fly through them without shields, and make it more difficult for weapon batteries to target ships behind them. Thus, the usual habit of firing weapon batteries first, then following with lances as the latter are not affected by blast markers.
At the end of each turn, rolls are made for repairs of critical hits and to remove blast markers that have simply faded out.
When ships lose half their hull points, they are considered crippled and most abilities (weapons, turrets, etc.) are halved. When a capital ship is destroyed, a roll for catastrophic damage is made, which most often turns the ship into a drifting hulk (cf. Space Hulk) but which occasionally detonates the ship's plasma drive or warp drive with bad results for any nearby vessels.
Any ship can ram another, but takes the risk of doing damage to itself in the process. Also, ships can board each other, using an easy formula plus a die roll to determine who wins the boarding assault and does critical damage to the ship. Many capital ships can also make Hit and Run attacks with their short-ranged teleporters to try to do critical damage to nearby opponents.
There are also rules for terrain effects, battle locales, gravity wells, solar flares, and easily accessible campaign rules and guidelines in the main book.
Currently, there are 9 races to play in BFG: Imperial, Space Marine, Chaos, Ork, Eldar, Dark Eldar, Necron, Tau, and Tyranid. Eldar also have a variation known as Craftworld Eldar. The two initial races are Imperial and Chaos, as those were the two key combatants in the Gothic War.
Imperial: Imperial ships tend to be somewhat slower than Chaos and have shorter-ranged weapons, but have better armor on the prow (so when they're in the best heading to be fired upon, the shooter is also facing their best armor) and are almost universally armed with torpedoes (giving them a powerful weapon to use as they approach with that heavily- armored prow.) Imperial ships are also the only fleet that has access to the devastating Nova Cannon, an inordinately long-ranged weapon capable of doing up to 6 hits on a target with a single shot.
Chaos: The Chaos fleet tends to be faster and carry longer-ranged weapon batteries and a plethora of attack craft. A typical game of Imperial vs. Chaos is also often torpedoes vs. assault boats, as the quicker Chaos ships try to stay out of close ranges (where torpedoes will strike without having to move counters) and hammer away at the enemy with their huge batteries, while the assault craft swarm the opponent under. Chaos also benefits from many slightly lower-priced cruisers in comparison to the Imperium and can take advantage of a few quirky concepts, like Daemon-possessed ships.
Ork: Ork ships are typically slower but hardier than other fleets (capital ships usually have 10 hull points, rather than 8) and their weapon strength and torpedo salvoes are randomly determined by die roll (potentially going from a full effectiveness 10 to a malfunctioning 5.) Orks have bonuses to boarding and benefit from the ability to field masses of cheap escorts (in true Ork fashion, numbers are key), as well as the only ship primarily designed to physically hit others, the Brute Ramship.
Eldar: Eldar use a special movement based on the solar wind (the direction of the nearest star being determined at the start of the game.) The positioning of their ships lets them move at slow, normal, and quite fast speeds, pending the direction of that star. Eldar ships also move twice, once in the movement phase and once in the ordnance phase. Eldar weapon batteries, lances, torpedoes, and attack craft are superior to most other fleets, allowing for re-rolls to either attempt to hit again, obviate misses, or keep attack craft in play that would otherwise be destroyed. Eldar also benefit from Holo-fields that let them ignore many attacks. The downside is that Eldar ships are quite fragile, having fewer hull points on their capital ships (typically 6), no shields (meaning they could always potentially take damage from things like Blast markers), and the lowest fleet-wide armor values in the game. If they get hit, they feel it, so the trick is to not get hit or destroy the enemy before he can hit back. The Craftworld variant is hardier and moves more slowly, but is essentially similar to the regular Eldar pirate fleets.
Dark Eldar: The Dark Eldar raiding fleets are similar to Eldar ships, but they don't rely on the solar wind and instead simply move quickly. Their weapons are similar to the Eldar and they use Shadowfields similar to the Eldar Holo-fields. However, the DE excel at boarding and assault craft over any fleet except the Tyranids, whereas Eldar will almost never board (considering it vulgar.) The fleet is also quite limited, with only one class each of capital and escort ships, but with varied weapon configurations for both.
Space Marine: Space Marines have a smaller list than the parent Imperial fleet, but their ships, while costly and short-ranged, are amongst the toughest in the game. Space Marines also excel at boarding and assault and their capital ships can also use the Bombardment Cannon, which fires like a weapon battery, but hits like a lance and has a greater chance of doing critical hits than any other weapon system. Space Marine fleets can often use Imperial ships drafted in for various missions, as well.
Necron: The Necrons tend to ignore many rules of the game and are quite tough, have powerful weapons and are, on average, the fastest fleet in the game. However, Necrons also have no shields, depending instead on saving throws (similar to, but less than, Brace for Impact), and are quite expensive in points cost. They also must deal with a victory point system that radically exaggerates the damage done to Necron ships, reflecting their desire to keep their technology out of the hands of any other race, no matter the cost.
Tau: The Tau fleet is kind of slow and their weapons often short-ranged, but their attack craft are among the best in the game, and they can perform excellent feats so long as the commander is willing to let his fleet work together and not depend on his capital ships to match up one-on-one with those of most other fleets. They also have multiple configurations for most of their major ship classes, which is something else that sets them apart from most of the other fleets.
Tyranids: Tyranids are slow, but benefit from easily and cheaply available escorts and hordes- HORDES -of attack craft. The Tyranid fleet is all about getting in close and assaulting the enemy, because no fleet is more damaging in assault and boarding. Many of their weapons get bonuses to damage at close range, and even the spore clouds which function as their shields can be used as weapons close-in.
Battlefleet Gothic is one of the best games that GW has ever produced and is also one of the most addictive. With the extremely different play styles of each fleet, trying new ones is always on the mind of the typical player. Furthermore, with the ease of creating names and back stories for one's capital ships, increasing the size of the fleets one already plays is just as inviting. One of the largest and most active Yahoogroups for GW games is for BFG and the residents (many of whom serve in the various naval forces of our own little planet) would love to tell you all that I've said and more: http://games.groups.yahoo.com/group/bfg-list/