Bilbo Baggins is the hero of J.R.R. Tolkien's The Hobbit.
The son of Bungo Baggins and Belladonna Took, he lived alone, after his parents' deaths, at Bag-End in Hobbiton in The Shire. Unlike most hobbits, who tend to be a prosaic race as a whole, Bilbo yearned for adventure. It came his way, when Gandalf involved him with a group of twelve dwarves, and their leader Thorin Oakenshield, rightful heir to the kingdom of the dwarves. Gandalf had convinced them that Bilbo was an acomplished burglar and identified him by means of a mark on the door of Bag-End.
Bilbo joined with Thorin's band in the quest for Erebor (The Lonely Mountain), the ancestral lands of Thorin's people, from where they had been driven 150 years earlier, by the dragon Smaug. Thorin was determined to have his revenge.
In the course of the quest, Bilbo beat the troglodytic Smeagol Gollum in a riddle game, and stole from him a ring of invisibility, which he later used to outwit menacing spiders. He identified the Dragon's vulnerable spot for Bard the Bowman, Lord of Dale, enabling the man to shoot and kill the last of the great firedrakes, so that the dwarves could reclaim Erebor, and Dale could be rebuilt in its shadow.
At the end of the quest, Bilbo returned home to Bag-End, and lived a peaceful life. He appears again in The Lord of the Rings and as the story begins, he is living with his orphaned nephew, Frodo, who he has adopted. However, the ring he had stolen has kept him youthful, arousing Gandalf's suspicions. Investigation proves that the Ring is the "One Ring" forged by Sauron in secret to rule the other nineteen great rings -- nine belonging to men, seven to the dwarves, and three to the Elves -- a ring which must be destroyed if Arda is not to descend into darkness.
Although Bilbo is reluctant to give up the ring, Gandalf coerces him into passing it to Frodo and to leaving the Shire, to live with Elrond, a half-elf, half-man (and the father of Arwen Evenstar) in Rivendell. Bilbo departs The Shire with style, giving away all his possessions, and holding a grand eleventy-first birthday party, with feast and fireworks provided by Gandalf.
This is the only major part he plays in The Lord of the Rings, but by surrendering the ring to Frodo, he makes possible all that comes after.
Bilbo spent the latter part of his life in Rivendell writing The Red Book of Westmarch (or There and Back Again) and The Road Goes Ever On and On, before departing from The Grey Havens to the legendary resting place of the elves in the West, a privilege granted only to the Ring Bearers (himself and Frodo, and later Sam Gamgee) among hobbit-kind.