Beorn is a man who lives beyond the Misty Mountains. He's a huge fellow, very strong, and he can transform into the shape of the bear (which makes sense, since björn is Old Norse for bear). Beorn is a little prickly and not too fond of strangers, but he absolutely, totally hates goblins. Thanks to his the-enemy-of-my-enemy-is-my-friend (phew!) logic, Beorn provides Gandalf, Bilbo, Thorin, and the other dwarves with a place to stay after they escape from the Misty Mountains.
He also wades in to shift the balance of the Battle of Five Armies towards the good side and against the goblins at the very end of the book. He fights in bear shape right through the goblin armies until he reaches the side of wounded Thorin. He carries Thorin out of the battle before continuing his rampage. Beorn's manner in battle is worth noting:
[After carrying Thorin off the battlefield] his wrath was redoubled, so that nothing could withstand him, and no weapon seemed to bite upon him. (18.23)
Beorn's killing rage sounds a lot like legends of certain Viking warriors, who would become so maddened during battle that they became invincible (now that's handy). These men were called "berserkers" – a word also related to the Norse word for "bear" (source). We don't think it's too much of a stretch to believe that Tolkien had berserkers in mind when he invented Beorn.
Despite the fact that he takes the shape of a bear, Beorn appears to be an animal-loving vegetarian. Beorn lives on bread and honey, which he collects from his hives of giant bees. And he's waited on by intelligent ponies, whom he loves like his children. And although he seems utterly comfortable living by his wild lone, the narrator assures us that Beorn goes on to become a great chief in the North, with generations of sons who are also capable of taking a bear's shape.