Counter-Strike: Source is a tactical FPS involving terrorists and counter-terrorists that has been re-made using the Valve Source Engine, based off the CS Half-Life mod.
Counter-Strike: Source is one of two major variants still in heavy use today, the other being the 1.6 version of CS. (It has stopped upgrading version numbers since 2003 when 1.6 came out, but is still updated via Steam, just new version numbers are not issued.)
CS:S was released in 2004 and was a major overhaul for Counter-Strike. Because the game used Valve's Source game engine, it allowed for the graphics and utilization of more modern audio hardware.