file is simply a renamed .ZIP file, used in games based on the Quake III
engine. It usually contains a checksum to prevent the opening and recompression of the file, or the addition of extra maps to demo versions. The .PK3 file contains the map, textures, weapons, sounds, and scripts for the game.
They can usually be viewed or extracted using a file archiver with support for the ZIP file format. One can also create .pk3 files using software designed for that purpose.
Quake IV, the sequel to Quake III, uses .pk4 files alongside the Doom 3 engine.