Do you remember those "How to Host A Murder Mystery" games from years ago? Each game in the series gave out roles to people, fed them information about themselves and the others in the group slowly, building up to the final scenes in which one person was revealed to be the murderer. The games were designed for large groups and could be played around a dinner party or other formal gathering.
Trouble was, you couldn't ever replay those games. Once you knew the murderer, that was it - send it to the thrift shop, get another one.
"Werewolf", in any of its various incarnations, is a cousin to those murder mystery games. Unlike its cousins, Werewolf is infinitely replayable, and can be scaled to nearly any number of players (from as few as 5 to as many as... well, how many people did you invite to dinner?).
In Werewolf, each player is handed a role card at random. Most people will end up being ordinary villagers, whose goal is to rid themselves of the murderous werewolves. A few players (generally 2 or 3) are the wolves, who know each other and spend their nights eating the villagers one by one. One player is generally a seer, who gets to check one player each night to see if he/she is a wolf or is harmless. With more experienced groups, other roles can be added in (such as a priest, a hunter, a bodyguard, a traitor, etc.). One person is the moderator, who keeps the action going.
During the day, everyone talks and makes accusations at will. They end the day by voting for one person, who ends up being lynched and out of the game. At night, everyone puts their heads down, closes eyes, and waits while the moderator allows players with night actions to run their course. The seer views a player, then the wolves choose a player to kill, and day comes again with the announcement of which person has been eaten by the wolves. When the villagers lynch all the wolves, they win - and if the wolves wear the villagers down to be equal with the remaining villagers, then the wolves win.
And that's basically it. No dice, no board, nothing but role cards (which you aren't even allowed to show anyone) and player interaction - lots of it.
Bringing out games to play with large groups can be difficult, especially when you have people in the mix who aren't big on playing games of any kind. But "Werewolf" is 99% guaranteed to have your dinner guests asking for more. (I reserve the 1% for those few people who hate having fun in general, not just hate playing games specifically.)
Werewolf is adaptable in uncountable ways and is suitable for nearly any group. Dinner parties are just a convenient excuse to play. It's also good for youth groups or any gathering of mid to older aged kids, camping trips, office parties, etc. Try substituting "Werewolf" for the typical "team building exercise" at your next corporate off-site meeting. You needn't use the "werewolf in a medieval village" theme, either. This game frequently goes under the name "Mafia" for example, where the wolves are "mafia hit-men" and everyone else is a "city citizen." Or try the Wild West, with a deputy and bandits. Nearly any theme could be slapped onto this game with absolutely no change in the rules.
The best aspect of the game is that you need not spend more than $2 to play it - if that. Although offical sets of role cards with nice artwork are available for purchase, you can easily download a set of printable cards from this site, or create your own. You can also use a standard deck of cards, assigning number cards as "villagers" and specific face cards as "wolves" and "seer", and so on. Or do what I did the first time I played with my youth group - write the roles on sticky notes or index cards.
You simply cannot go wrong with having this game on hand for a large group.