Here you will find a list of the confirmed official maps for Counter-Strike: Global Offensive. A total of 16 maps will ship with the game on release. This includes 8 classic map remakes for the casual and competitive game mode and 8 new maps for the Arsenal game mode (the mode based on the popular Gun Game mod).
There are several different map types for CS:GO. There is the map type 'de_' which refers to the bomb plant / defusal maps in classic competitive and classic casual game modes. In these maps, the objective for the Terrorists is to plant the bomb at one of two designated bomb sites and stopped the Counter-Terrorists from defusing the bomb. Likewise, it is the Counter-Terrorists objective to stop the Terrorists bomb plant or defuse the bomb before it goes off. The 'de_' map type is probably the most played map competitively.
Another map type is the 'cs_' maps, they ususally refer to hostage maps but not always. In the hostage maps the Terrorists objective is to stop the Counter-Terrorists from rescuing the hostages. Obviously then, it is the Counter-Terrorists objective to save the hostages and evacuate them to an extraction point, usually being the Counter-Terrorists spawn location.
Other map types are the Arsenal maps, also dubbed as 'ar_'. These maps are used in the Arsenal arms race game mode. 'de_' is also in the Arsenal demolition game mode.