A WAD (Where’s All the Data) is the file format id Software invented for Doom in 1993. A single .wad file packs together everything a level needs: map geometry, textures, sprites (the 2D images used for enemies and items), sound effects, music, and (in some cases) code.

There are two kinds of WAD:

• IWAD (Internal WAD) — the base game data, like DOOM.WAD or DOOM2.WAD. You need to legally own a copy.
• PWAD (Patch WAD) — everything else: community maps, mods, mapsets. PWADs override or add to the IWAD.

Source ports load WADs in the order you pass them on the command line, with later files taking precedence. This is how modding works in Doom: you stack a PWAD on top of an IWAD.