The game has been expanded into a series since its original release, which currently includes Counter-Strike: Condition Zero, Counter-Strike: Source, and Counter-Strike: Global Offensive. Counter-Strike pits a team of terrorists against a team of counter-terrorists in a series of rounds. Each round is won by either completing the mission objective or eliminating the opposing force.
The game was the most played Half-Life modification in terms of players, according to GameSpy in 2008.
As of August 2011, the Counter-Strike franchise has sold over 27 million units.