Counter Strike 2009 is a simple team-based FPS (first-person shooter) in which opposing teams of Terrorists and Counter-Terrorists have to try and wipe each other out whilst managing map-specific objectives, like rescuing or protecting hostages and planting or defusing bombs. Each round is slightly more strategic than something like Capture The Flag though, because whenever you die you actually remain dead - until one team is wiped out completely, or until one team completes its objective.
Throw in instant headshot kills, a carefully balanced arsenal and weapon limits, monetary rewards and punishments for kills, hostage rescues, hostage kills, etc, and consistently ingenious map design, and its not too hard to understand Counter-Strike's popularity. It demands proper teamwork and less gung ho Schwarzenegger antics than its rivals, and although not quite perfect it is extremely well balanced and finely tuned, and arguably represents the pinnacle of team-based multiplayer even now. Regularly threatened by an insurgent flavour of the month FPS, Counter Strike still remains the most popular game in its genre, and is fervently supported all over the Internet.