Berserker is the
anglicised version of the
Norse word, Berserkr, although the Norse word didn't mean this (I shall explain later). Berserkr began as a
myth, he was known as a warrior that was virtually
invulnerable to anything that the enemy could throw at him. Whether it be arrows, swords, axes, hammers, flame - it mattered not, all fell before the might of this warrior. He wore no armour and simply defeated his enemies with the sheer
verocity of his
rage. His powers were due to him being the son of two gods,
Starkadder and
Alfhilde. What is quite interesting abour the
tale of Berserkr, is that this rage transformed him into a
wolf or a
bear, meaning that this tale was the first ever tale of
lycanthrope.
As you might expect, the Norsemen found this warrior quite appealing, and they began to mimick him to some extent. While there are tales told by Romans of "crazed" Germanic tribesmen who wore animal skins, the actual Berserkr wasn't really hatched until the Viking invasion of Brittania. The Romans definently met individuals trying to mimick this warrior, but the true image of the Berserkr came into conception with the Viking Berserkrgangr. The Berserkrgangr, literally meaning bear goer (this is where to tale of lycanthrope comes into it again), was a large mass of these Berserkrs with no set numbers and no formations. Viking commanders did not command them in the least, but simply let them ravage a battlefield.
The Berserkr generally wore a loin cloth and generally the skin of a bear or a wolf, although it was not unheard of them using other animals on other occassions. The weapon of choice for most Berserkrs was the Viking two-handed axe, due to its power and the ease of swinging it about at great speeds. The next most common weapon was a simple one-handed axe, of which they would either use two or one with a small shield. Other than axes, no other weapons were used by the Berserkr as they were more difficult to swing strong and quick.
Tactics were non-existant, but there was a general method used by the Berserkr. Firstly they would often get drunk and brawl with each other to raise adrenalin and numb their nerves, then they would eat halucinogenic mushrooms, and often drink some more, before going into battle. When they did enter battle, they were generally so off their face that they can't even feel the fact that they are standing barefooted on snow (a lot of Viking battles were in cold areas). When battle did commence, it was usually because one Berserkr saw the enemy and charged, bringing the whole lot of the Berserkrgangr with him. The Viking commander now had the choice of letting them go unsupported until he was ready, or following them into battle, but he had to consider that even his cavalry (who rode on Scandinavian ponies - not the fastest things around) would have trouble keeping up with the Berserkrgangr. They were incredibly fast, as they were high on adrenalin, alcohol and hallucinogenics, and carrying virtually no armour.
Once the Berserkrgangr was in action, it was incredibly effective, but it didn't last long. Think of this situation: you are a British peasent, and you just got yanked off your farm because some crazy folk from Scandinavia just sacked a few Northumbrian cities and pillaged, and now they've come after your lands. You're already terrified, you've never seen a Viking before, only heard tales. Now you enter the field, you see the Viking army stretched out, and then all of a sudden a whole mass of god knows how many Vikings, wearing virtually nothing, bellowing at the top of their lungs, looking like madmen and running full bolt at you, swinging their axes, without even considering formation. Now what the hell are you going to do? Most likely run as fast as you can.
This was the effect of the Berserkrgangr, so much so that even the most disciplined troops would find themselves unnerved. Once they actually reached the enemy, they would attack constantly and swiftly, without regard to their own life. In fact, they were so inebriated that they wouldn't feel the wounds done by the enemy, and they were very slow to tire. Only problem was, they didn't last long against truly elite troops. If a disciplined unit managed to hold formation, a few careful hits could take down an unarmoured Berserkr, while a heavily armoured Englishmen was going to take quite a few hits before he is taken out. Another problem was that they really didn't stand a chance against spearmen, so long as they managed to keep hold of their spears, and the Berserkrs weren't really the type to bother discerning between enemies - they were all there to sacrifice to Odin as far as they were concerned.
The final weakness of the Berserkrgangr was that once they were in the fury of battle, things more often than not got out of hand. Berserkrs not in the front row of the mass might get so caught up in the battle that they fail to recognise other Berserkrs, and would start swinging wildly, causing the other Berserkrs to do the same. Also, a Berserkr would never retreat, they would fight until the very last straw, and either win as a whole or die as a whole. The average life of a Berserkr was usually one battle, and it was all too possible for their death to be caused by one of their own. Berserkrgangrs were still quite common in the Viking invasion of Brittania, and it wasn't as if a Viking commander could really prevent them from coming over even if he had wanted to. They were an incredible fighting force nevertheless, lowering morale, often causing less disciplined troops to route, and often dealing extensive damage.
Sources:
http://www.geocities.com/fullmoonsite/berserkr.htm
http://www.florilegium.org/files/NORSE/Berserkergang-art.html