"Doom WAD is the default format of package files for the video game Doom or its sequel Doom II, that contain sprites, levels, and game data. WAD stands for "Where's All the Data?" Immediately after its release in 1993, Doom attracted a sizeable following of players who created their own mods for WAD files—packages containing levels, graphics and other game data—and played a vital part in spawning the mod-making culture which is now commonplace for first-person shooters. Thousands of WADs have been created for Doom, ranging from single custom levels to full original games; most of these can be downloaded for free over the Internet. Several WADs have also been released commercially, and for some people the WAD-making hobby became a gateway to a professional career as a level designer.

"When developing Doom, id Software was aware that many players had tried to create custom levels and other modifications for their previous game, Wolfenstein 3D. However, the procedures involved in creating and loading modifications for that game were cumbersome. John Carmack, lead programmer at id Software, designed the Doom internals from the ground up to allow players to extend the game. For that reason, game data such as levels, graphics, sound effects and music are stored separately from the game engine, in "WAD files". This allowed players to make their own data without any modification to the engine itself."

Description from Wikipedia

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In the rush to fill CD-ROMs with collections of shareware, vendors began acquiring Doom WADs at great speed, putting together compilations of levels, along with additional graphics, reviews, documents, and other related files. With the introduction of Doom II and Quake, custom levels, tools and assets continued to be made available and sold on these CD-ROMs until the fervor for CD-ROMs faded in the 2000s. This collection contains many thousands of user-designed levels across many years.