VMF

Valve Map File

A proprietary map source format used by the Valve Hammer Editor to store both maps and prefabs. It contains information on all brushes and entities in a script format.

VMF files are written in the same KeyValues text format that the engine uses to parse external configuration files, this allows them to be opened and edited in plain text.

Full documentation on a vmf file's contents can be found at the VMF documentation page.