Ok, my friends have played this game since it was in beta testing (summer 1999). I watched them, but I never played -- until about 2 months ago. This is one of the best, most addicting games ever. Counter-Strike is a multiplayer-only PC game played either over a network or the Internet.
The game simulates terrorist/counter-terrorist action. You can join either the terrorist or the counter-terrorist (CT) team. The two most common scenarios are bomb defusion and hostage rescue. In bomb defusion, one terrorist is given a bomb and must plant it at a specified location. The CTs must either kill all of the terrorists prior to the planting of the bomb or defuse the bomb once it has been planted. In hostage rescue, the terrorists must prevent the CTs from rescuing the hostages.
A little background into the making of the game: Counter-Strike is a modification of Valve Software's game "Half-Life." Counter-Strike was not created by Valve, but by a group of people seeking to improve the multiplayer aspects of the game. It is more popular than the original game, played by millions of players all over the world.
Note: This game features graphic violence and should not be played by children.