From Quake Wiki
A .wad is best described as a stand alone software container that holds several textures at once (up to 800?). It's primary function is to be used during the map making and compiling process. The term "Wad" comes from it's software extension (eg: Id_Base.wad).
There are several variations of .wad files. For example a Doom level is also called a .wad file. There are also .wad files that perform the same function as a Quake .wad but are in different formats for different games, that are not compatible with the Quake standard. For example Half-Life (whose game engine is based upon the Quake game engine) also uses the extension of .wad.
There are many free software tools available to create and\or manage wad files that will extract the textures inside the original game and put them in a proper Quake .wad file. One of the most popular is TexMex.
There are also many pre-made .wad files available on the internet.
quaddicted.com/files/wads - exhaustive collection of pre-made .wad files
|Hectate added a To-Do; Detail out some of the free tools available for .wad editing, and places to find pre-made .wad files as mentioned above..|