Counter Strike: Source

Counter-Strike: Source games pit a team of counter-terrorists against a team of terrorists in a series of rounds. Each round is won by either completing a mission (such as detonating a bomb or rescuing hostages) or by eliminating all members of the opposing team.

The Omniscient Wikipedia has a pretty good description.

If you enjoy playing CS:S and attend ACM’s LAN parties, you might consider using the ACM tag.

Example: <ACM>JohnDoe

• CS:S Strategy Guide - Gamefaqs has a good breakdown of weapons, stock maps, console commands, and mission objectives.