The Berserker powerup in Doom 3 has a lot of things in common with its classic counterpart the berserk pack. However, the berserker only has an effect for a limited duration and also makes the player invulnerable against any monsters' attacks as well as the enhancement of melee attacks and speed (as perceived by player). It enhances the damage far beyond what the classic powerup did, enabling the player to kill even the toughest non-boss monsters with a single punch (while in Berserker mode, a single punch deals 1020 damage). There is no major alteration to health and armor (if any) after the beserker effect wear off.
One must also note that, the invulnerability of the berserker doesn't protect the player from any damage other than attacks from monsters. that is, a player with berserker will still take damage or die from his own rocket/handgrenade explosions and bfg-overloadings, explosive barrels and other environmental damages/deaths, falling damages, and multiplayer opponents' attacks.
The item for the powerup appears to be a floating skull equipped with a standard marine helmet with added demonic motif and blazing red aura surrounding it. This, combined with the constant screaming sound while armed, makes it possible the helmet is a demonic technology. The Berserker is not often found in the game and is extremely rare, only being found in Communication Transfer and in Hell. The Berserker in the Communication Transfer, meanwhile, is seemingly only there as a "demo", with the player being ambushed by several zombies after picking it up but unable to leave the area and progress on until after the Berserker powerup wears off.
In the expansion, the Berserker does not appear in single player at all, being replaced by The Artifact, but appears in multiplayer.