PWAD

A PWAD, short for patch wad, is a WAD containing lumps of data created by a user as an add-on. PWAD lumps are given priority over IWAD lumps in order for PWADs to replace anything that was included in the original game.

The PWAD is the most familiar and widespread type of add-on for the Doom games and is typically used to store custom maps, define additional textures, or override or add to the default textures, sprites, sounds, or other data provided by the game's IWAD.

When the game is run, it loads one IWAD and can be instructed to load one or more PWADs along with it with the  parameter. After resources contained in the IWAD are loaded, data from the PWADs are loaded, replacing or adding to any resources that were already defined by the IWAD. In a typical usage, a custom map stored in a PWAD will be used instead of the game's default map for that level. The PWAD may also define wall textures, flats, and almost anything else the map needs, and the unspecified data will be loaded from the original IWAD. By defining only the information not already present in the IWAD, PWAD files can be much smaller than a full IWAD, and allow users to distribute new content without including much, if any, of the original work from the games.

The aggregate number of PWADs in existence is indeterminate, though a five-figure total seems likely, given the reported size of a single archive, idgames. Most of these can be downloaded for free, although a few have also been released commercially. For some people, the WAD-making hobby has become a gateway to a career as a game designer.

Source

 * How many Doom wads made so far?, thread at the Doomworld forums