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.
PWADs are typically used to store custom maps, define additional textures, add player skins, or override the default textures, sounds, or other data provided by the game's IWAD.
When the game is run, it loads one IWAD and has the option of loading one or more PWADs. After resources contained in the IWAD are loaded, data from the PWADs are loaded, replacing 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 and almost anything else the map needs, and the unspecified data will be used from the original IWAD. (In vanilla Doom, replacing ceilings and floors, known as flats, can be problematic). By defining only the information not already present in the IWAD, PWAD files can be much smaller than the full IWAD.
The aggregate number of PWADs in existence is indeterminate, though a six-figure total seems likely, given the reported size of a single archive, Doomworld/idgames. Most of these can be downloaded for free, although a few have also been released commercially. For some people, the WAD-making hobby became a gateway to a career as a game designer.