What is a WAD?

The Quick Answer

When talking about Doom and some of its relatives, a WAD is a type of data file. These files use a simple format to store the assets (maps, graphics, music, text) that the game needs. On older computers, filenames could only have an 8 character name with a 3 letter extention. WAD files, therefore, used WAD as their extention.

The term "WAD" is said to originate from the phrase "Where's All the Data?", which is another way to remember what sort of information the files contain.

Types of WADs

The WAD format specifies two different types of WADs: IWADs and PWADs. At least one IWAD is required to play the game; PWADs are optional and are usually reserved for adding custom content to the game.

IWAD

Also known as "internal WADs" or "initial WADs", these contain the default assets for the game. Most IWADs are not legally available for free - you'll need to purchase them by buying a copy of Doom or Doom 2. The main exceptions to this are the IWADs for free games that use the Doom engine, like FreeDoom or Chex Quest.

PWAD

On the other hand, PWADs or "patch WADs" contain new assets that override the default content when they are loaded into the game. These are typically distributed for free online, as they are the files that contain the new maps and levels that players create.