Counter Strike is a multiplayer first person shooter, that began as a Mod for the iconic Valve game Half-Life.
Having started out back in 1999, and last updated in 2003, Counter-Strike is far from a cutting edge FPS game. Despite its age, and much like ID's Quake, the game survives due to its essential playability. It may not be sophisticated, but Counter-Strike is fast, fun to play, and has a massive community of players worldwide.
Counter-Strike is played over a variety of maps, and is a team based multiplayer FPS. Players either join a team of terrorists or counter terrorists, and battle it out. While it's a genuine video game classic, it's hard to recommend Counter-Strike except out of nostalgia. Call of Duty and Battlefield do online multiplayer much better today, with better game modes, and of course much better graphics.
Counter-Strike is a classic FPS online multiplayer FPS, and deserves its place in history, but is difficult to recommend to today's' gamer.