Maps in Doom 3 are defined by four different files, all of which are in ascii so it should be very easy for other people to write editors and tools for them. For a map to work properly, all four files must be included (the only exception is .aas files are not needed for multiplayer maps).

.map
The .map file is the main file that is created when you edit a file, it defines all the entities and brushes in the map. The other three files are all generated from the .map file with the dmap command. The format hasn't changed much from the Quake series.

.cm
The .cm file defines the collision geometry in the map. It is used by the physics system for collision detection.

.proc
The .proc contains all the pre-processed geometry in the map. It stores all the visible triangles, batched up in to surfaces. It also stores all the portal information, and any precalculated shadow volumes (if a light doesn't move, and a brush doesn't move, the shadow volume can be precalculated).

.aas
The .aas files contain the 'area awareness' data for the AI to navigate through the level. A seperate aas file is generated for each size monster. Generally an aas48 and an aas96 file is generated for most monsters sizes. If a map has a special monster in it, such as the mancubus, saboth, guardian, or cyberdemon, then it will generate a special aas file for them.