"doomarcade wad"? Can someone enlighten me?
Games in the Quake/Doom family use what's called a .wad file to store things like maps, graphics and other game related information. Certain games use the wad files a little bit differently. For instance, the Half-Life line uses the format to store textures during map creation. Things like maps, models, sounds etc for Half-Life are stored either in the older pak format or in the new gcf format. In fact, I think the newest gcf stores the wads, I'll have to check.
I don't know exactly which wad he's talking about since a search turns up a couple of potential canidates. But basically you can download .wad files other people create and install them (of sorts) into the game thereby adding new levels, or even entirely new gameplay utilizing an existing engine.
I don't play with the Doom line very much so I don't know how extensive the role the wad file plays. I spend more time with the Half-Life branch.