In the game Quake the Quad gives you four times the firepower. This allows you to gib your foes with the greatest of ease. The Quad damage item in Quake III is actually a misnomer, as it grants the player only 3 times the normal firepower.
When the Quad is attained in a team-multiplayer match, it is lost when the person who got it dies - it does not fall to the ground as an item again.
Quad damage is sometimes disabled in multiplayer games because it unbalances the game.
For the unskilled, or just unlucky player, the Quad can potentially be a liability. Because Quad damage affects the damage of explosive weapons as well, a player can much more easily kill themselves with their own splash damage.