Short answers, pulled from the story.
The Old Norse word berserkr is a compound of ber and serkr. Most scholars interpret ber as meaning "bear," making berserkr mean roughly "bear-shirt." The 13th-century historian Snorri Sturluson proposed "bare" instead, suggesting warriors who fought without armour, but that interpretation has largely been abandoned for lack of supporting evidence.
Berserkers were outlawed in Iceland in 1015. That same year, Jarl Eiríkr Hákonarson of Norway also outlawed them. The medieval Icelandic law code Grágás sentenced berserker warriors to outlawry, and by the 12th century organised berserker war-bands had disappeared entirely.
The earliest surviving reference to the term berserker is in the Haraldskvæði, a skaldic poem composed by Thórbiörn Hornklofi in the late 9th century in honor of King Harald Fairhair of Norway. The poem refers to them as wolf-skinned warriors who bear bloody shields and hack through enemy shields.
The sources describe berserkergang beginning with shivering, chattering of the teeth, and a chill in the body, followed by swelling and colour change in the face, then escalating into howling, shield-biting, and indiscriminate violence. Proposed causes include hallucinogenic mushrooms, large quantities of alcohol, a substance called butotens, self-induced hysteria, epilepsy, and mental illness. Seeds of black henbane (Hyoscyamus niger), whose symptoms resemble those of the berserker state, were found in a Viking grave near Fyrkat, Denmark in 1977.
Berserkers were associated with the bear cult and were said to shapeshift into bear form, while ulfheðnar were wolf warriors who wore wolf pelts over their chainmail in battle. Ulfheðnar appear in the Vatnsdæla saga, Haraldskvæði, and Grettis saga and are consistently described as the elite following of King Harald Fairhair. Direct references to ulfheðnar in the sources are scarcer than those to berserkers.
J. R. R. Tolkien drew on Norse mythology in his Middle Earth tales, including The Hobbit, where the character Beorn is a berserker who can transfigure into a massive bear, dangerous to both friend and foe.