"Where's All The Data?" – id Software

A WAD file is a file that contains data as its name suggests. The type of data it supports differs by version:

• WAD1 for Doom (id Tech 1) contains the majority of game data.
• WAD2 for Quake (id Tech 2) contains textures and graphics data, with the majority of data moved to PAK files.
• WAD3 for Half-Life (GoldSrc) is an extension of WAD2 where each textures has its own colour palette.

WAD3/GoldSrc

In GoldSrc, WAD3 files contain mostly textures used in maps. In the map editors (e.g. Hammer Editor or J.A.C.K.), you set a list of WAD files to be used, and the compiled .bsp files references these files. The engine then looks for the wad file names when looking for textures (if it's not embedded in the bsp file itself).

There are also specific wad files that contain non-conventional texture data:

Name	GoldSrc Version	Description
decals.wad	WON & Steam	Contains textures that are used by decals, where the texture data is grayscale except the last palette index which specifies the chroma colour.
cached.wad	WON only	Contains the loading graphics and the drop-down console background.
fonts.wad	WON only	Contains rasterized fonts used for console texts, chat, and killfeed.
gfx.wad	WON & Steam	Contains "LOADING" and "PAUSED" graphics (WON & Steam), and some more fonts including the credits font (WON only)
tempdecal.wad	WON & Steam	Contains the currently selected spraypaint, and overwritten with new selection.
spraypaint.wad	?	Some small collection of spray paints, probably deprecated in favour of plain .bmps in logos/ and the generated tempdecal.wad.

The Tools and Resources page contains a list of programs that can open WAD3 files. You can also read the specification for WAD3 for technical details.