The following tutorial describes 6 most common file types that you will encounter while using Hammer Source and SourceSDK/Authoring tools.
It's important to know these file types are and their purpose.
6 Common File Types are:
.bsp - file extension for maps/levels
.vmt - Valve Material Type. Text file that contains information for Source Engine to simulate surface types.
.vtf - Valve Texture Format. Texture format used by Source Engine.
.nav - Navigation Mesh file for bots. Walkable space in the map for bots to navigate around.
.vpk - Valve Pack File. Uncompressed archive used to package maps content. Such as campaign files for L4D2.
.vmt - Valve Map File. Raw map file data. File type used in Hammer Source Editor. This contains all the entities, brushes and textures that you see in Hammer Editor.
Additonal file types include:
vtx, mdl, phy, ani, vvd, qc, smd