"Master Levels for Doom II is an expansion pack for Doom II which was released officially on December 26, 1995 by id Software. The CD contains 20 WAD files created by various authors under contract. The file teeth.wad contains a secret level, so there are a total of 21 levels." (copied from Wikipedia)
You don't actually need the Doom II game to play these levels; these also work perfectly with Freedoom.
The levels were originally started with the doomit.exe utility that let you browse the levels available and read their individual documentation sheet.
The doom2-masterlevels utility aims to provide the same functionality on modern systems. It will also provide a link to individual pages on DoomWiki with updated info about each maps.