Who is Beorn?
Beorn has the capability of skin-changing, or you could also know this as morphing. He can change from a black bear to a great strong black-haired man with huge arms and a great beard. He is very fond of his animals and cares for them greatly. He doesn't like it if you talked to him about anything that could come out of a dead animal; clothing, shoes, tunics, carpets, furniture, cape, tippet, muff, rug.
Coming from the book you can state that Beorn could be a descendant of the first men that lived before Smaug or other dragons arrived into that part of the world. He could also be a descendant of the great and ancient bears of the mountains that lived there before any of the giants got there. He can talk with animals, and some work for him in his great wooden house, located in a forest of oak trees.
He built the steps on the great rock, he calls them by the name of "the Carrock".

How does he help the dwarves, Bilbo, and Gandalf in their quest?
Beorn helps them in their quest by aiding them with information about the location of Mirkwood, and where they should go by if travelling there. He gives them ponies so they could travel on and provisions so they are able to eat while they go into Mirkwood. He also gives them some advice about which way they should travel once inside Mirkwood and what they shouldn't do once they are inside.