Skulltag is a Multiplayer add-on for DOOM that runs on a modified ZDoom.EXE. It is basically its own game, although a WAD (data) file from a purchased version of DOOM or DOOM 2 is required to run it.
The basis of Skulltag is like that of Quake 3 and Unreal Tournament: Deathmatch and Teamplay games, such as Capture The Flag.
Weapons added to Skulltag (along with the standard Doom weapons) are:
Minigun- Generic "Spray and Pray" weapon, requires time for barrels to speed up/slow down.
Grenade Launcher- Somewhat like the Grenade Launcher in the Quake series
BFG 10K- Based on the Quake 3 weapon.