A PWAD, short for patch wad, is a WAD containing lumps of data created by a user as an add-on. PWAD's 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 or floor textures and anything else the map needs, and the missing data will be used from the original IWAD. By defining only the information not already present in the IWAD, PWAD files can be much smaller than the full IWAD.
The WAD format is not adapted to the needs of large mods. With its flat structure, easily-broken namespace markers, and limited naming possibilities, it is inconvenient to maintain during the elaboration of a mod. It is also more resource-intensive as far as ZDoom's archive manager is concerned, as the entirety of the file must reside in memory for its resources to be accessed. While this isn't a problem as far as most mods are concerned, some modern mods (for example, Æons of Death) can weigh several hundred megabytes.
For these reasons, ZDoom supports several formats of compressed archives. Using ZIPs as WAD replacement allows to create archives that are easier to maintain for the mod-maker, and less strenuous on the system for the mod player.