Counter-Strike pits a team of counter-terrorists against a team of terrorists in a series of rounds. Each round is won by either completing the mission objective or eliminating the opposing force. The latest incarnation of the game, Counter-Strike: Source, is based on the Source engine developed for Half-Life 2.
The game has both individual and multiplayer options. Bots can be added into the game. The online-multiplayer option is activated via Steam.
Other than similarities in basic gameplay, Counter-Strike is not in any way related to Half-Life. The original version was marketed as a commercial mod, but since then it has grown into a separate game in itself. The most recent versions do not require Half-Life to play, unlike a mod, and are no longer marketed as Half-Life: Counter Strike.