Counter-Strike began as a multiplayer mod to the Half-Life game engine. It is a team-based FPS with two sides consisting of terrorists and counter-terrorists. There are two main ways to win a game of Counter-Strike. You can either complete a mission, various examples of which include bombing a site as the terrorists or rescuing all the hostages as the counter-terrorists; or you can simply kill all the members of the opposing team.
The game went through 6 different versions before being ported to the "Source" engine in 2004. This greatly improved graphical details to the gameplay as well as letting more players play in a game. Counter-Strike 1.6 (the final version of Counter-Strike prior to source being released) is currently the most played game on the popular "Steam" client provided by Valve.
Popular maps include de_dust2, cs_office, "glass maps" which consist mostly of windows which are able to be shot causing the player to fall into a giant hole, "surf maps" which make use of a glitch in the game which allows a person to "surf" on a wall down a maze of sorts to the weapon array (players start with only a knife and there is generally no buy zone in these types of maps).