Custom maps made by the Doom community for Doom games. Original games released by id Software in the '90-s had the IWAD format and serve as asset sources that custom map sets are built upon. Those custom maps use the PWAD format, hence the group name that makes it clear it's not made by id.
See also:
Note: When adding game resources links to individual PWADs, only make them link to GameResources/DoomEngine in order to avoid infinite clutter in the header for GameResources/DoomEngine/Doom.