Counter-Strike: Source (CS:S) is a team based First Person Shooter which is a port of the original Counter-Strike mod for Half-Life in the Source engine that was used in Half-Life 2. The game was originally a total conversion mod made in the GoldSrc engine. All the Counter-Strike games pit a team of counter-terrorists against a team of terrorists in rounds of competition won by completing an objective or eliminating the opposing team. A more in depth explanation on the rules and gameplay of CS:S can be found on the Counter-Strike page.

Taken from Wikipedia