A .PK3 file is 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 maps, models, textures, weapons, sounds, scripts and other assets for the game.
They can usually be viewed or extracted using a file archiver with support for the ZIP file format. PK3 files can be created using software designed for that purpose.
Quake Live also uses PK3 files, but these files have been encoded by means of a bitwise XOR with a string of characters and therefore cannot be opened with a regular ZIP program like other PK3 files. However, open source utilities have been created to decode these PK3 files, such as the quakelivedec program.