.MAP files are a type of debugging symbol.
These are typically plain text files that indicate the relative offsets of functions for a given version of a compiled binary.
.MAP files are also uncompiled Quake maps. These files are plain text files. Detailed file format specifications can be found on the internet: Quake MAP Specs
In addition, the Halo series uses .map files for the levels. Duke Nukem 3D uses .map files for user-created levels.
.map files are also used by the MapInfo Professional Geographic Information System.
Another form of the .MAP file is for HTML image maps. An image map is formatted in HTML and creates click-able areas over a provided image.