*Maps*

The maps in Counter-Strike determine the gameplay and mission objectives. There are three official types of maps in Counter-Strike:
Hostage rescue maps, identified by "cs" before the name (eg. cs_militia, cs_office), where the counter-terrorist team's objective is to free several non-player characters that are present near the terrorist base
Bomb defusal maps, identified by "de" before the name, (eg. de_dust, de_aztec), where the terrorist team must try to plant explosives at specific sites in the map and protect it for a set amount of time while the counter-terrorists attempt to defuse it
Assassination maps, with an "as" prefix, (eg. as_oilrig), in which one member of the counter-terrorist team becomes a VIP, and must be protected from assassination until he reaches an escape zone. This game mode is not present in Counter-Strike: Source.
In earlier beta versions of the game another official type called Terrorist Escape also existed and, prior to commercial release, there was a racing map, where player-controlled vehicles were tested and eventually discarded.
Players also have the ability to create their own maps using the Valve Hammer Editor.