In short: The Berserker can fight longer, harder and more savagely than other fighters because he enters an ecstatic state of mind. This makes him a deadly warrior, who can be as much a menace to himself as to his enemies. The Berserker abhors ranged combat, preferring to fight hand-to-hand. When he enters his berserk state, he is immune to mind-affecting spells like Charm Person, gets improved saving throws against others such as Hold Person, and receives +1 to attack rolls, +3 to damage rolls, and +5 hit points.
While berserk, he must fight each opponent until they are down on the ground, must attack the nearest opponent each time, cannot be healed and cannot use missile weapons. After leaving the berserk state, the character loses the 5 temporary hit points and is exhausted. Dwarven fighters taking this kit will be called Battleragers. You will be able to decide when you go berserk, like Minsc was able to in the original Baldur's Gate.
Requirements: Strength: 15
Description: The Berserker can achieve an ecstatic state of mind in combat that will enable him to fight longer, harder, and more savagely than any human being has a right to. In his tribe, the Berserker has a special role. He has been touched by supernatural forces, and accepted that touch so that he might better defend his people. The idea of a Berserker Paladin is a little strange, and some DMs might not allow it. However, if the character's tribe is deeply involved with an appropriate animal totem, such as a bear or wolverine, a paladin might even be required to be a Berserker.
Weapon Proficiencies: No specific weapon proficiencies are required of a Berserker, but he may not start out play having a proficiency in a ranged weapon (no thrown axes or knives, no bows or crossbows, etc.). The Berserker lives to destroy things in hand-to-hand combat, so he cannot start with any sort of ranged weapon proficiency.
Armor/Equipment: A Berserker may not use his starting gold to buy armor heavier than splint mail, banded mail, or bronze plate mail. Once he has adventured in the outer world he can use any type of armor without penalty. When he spends his starting money he must limit himself to weapons known to his tribe, and may not choose missile weapons. Good choices include battle axe, club, dagger or dirk, footman's flail, mace, or pick, handaxe, spear, or sword (any type).
Berserkers receive a +3 Reaction Adjustment bonus from NPCs belonging to any tribe that also has Berserkers - they recognize the Berserker instinctively and respect him, even if he is an enemy. The other benefit the Berserker receives is his Berserk. At any time, the Berserker may choose to Go Berserk. This isn't an instantaneous process, he must spend a little time to "psych himself up". It takes a full turn to Go Berserk.
In that time, the character is growling, moaning, uttering imprecations - it's impossible to be quiet when trying to Go Berserk. He may also be fighting during that time, meaning that he can start to Go Berserk on the round the fight begins, fight for ten full rounds, and then be Berserk on the eleventh round.
The Berserker can also Go Berserk before the fighting starts. At the end of a full turn of preparation he can become berserk instantaneously. If there's no enemy in sight yet, he can hold the Berserk until combat is engaged. But if no combat takes place within five more full turns, he automatically reverts to "normal" and suffers the ordinary consequences for coming out of Berserk. The character can come out of Berserk once the last enemy is down (he must literally be down on the ground, even if still alive and surrendering). The Berserker will stay berserk as long as there are enemies still on their feet.
For these reasons, Berserking is a more appropriate reaction when the characters are about to attack a foe they know about. If the characters are, instead, jumped by a small party of orcs, it's usually not worth the effort to Go Berserk; the consequences and effort outweigh the benefits. When Berserk, the character has phenomenal endurance and resistance to pain and some forms of magic. Only while Berserk, he gains the following benefits:
Races: It's the DM's choice as to whether his demihuman characters can have Berserkers amongst them. It's entirely appropriate for dwarves, and not inappropriate for elves, gnomes and half-elves. Halfling Berserkers are not very likely. In any case, Berserkers wouldn't advertise the fact that they are such; until the first time they berserked in combat, their companions would probably be unaware that they are Berserkers.
Source: The Complete Fighter's Handbook