Counter-Strike is a modification to the excellent game, Half-Life. It modifies the multiplayer aspects of Half-Life to allow more team-oriented gameplay. Like Team Fortress2, in Counter-Strike the player experiences what a trained counterterrorist unit or terrorist unit experiences.
One team plays the role of the terrorist unit and the other team plays the role of the anti-terrorist unit. Each side has access to different guns and equipment, and different abilities. Maps will have different goals, such as hostage rescue, assassination, kill the entire opposing force, and more.