"Continuing with Valve's policy of orienting each episode around a particular theme or set of technologies, Episode Two aims to focus on expansive environments, travel and less linear play. Following the closing events of Episode One, it sees Gordon Freeman and the series' other major players moving away from City 17 to the surrounding countryside.
Like its predecessors, Episode Two has the player navigate Gordon Freeman through a linear set of levels, fighting off transhuman troops known as the Combine as well as hostile alien creatures. Puzzles and sequences involving vehicles are interspersed throughout the game, breaking up moments of combat.
One of the focal points of Episode Two was meant to be increased use of vehicles in open areas. However, the game retains its original linear style until the final battle.Episode Two has more puzzles than Episode One, including the biggest physical puzzle yet in the series a damaged seesawing bridge.The game features numerous "achievements" (similar to Playstation 3's Gamer Level and Xbox Live's Gamerscore) for carrying out certain tasks. Some are essential to game progress, such as helping fight off an antlion invasion, or defeating the first Hunters. Others are optional tricks or feats the player can perform, such as killing a Combine soldier with their own grenade or running down a certain number of enemies with the car."
Source: Wikipedia, "Half-Life_2:_Episode_Two," available under the CC-BY-SA License.