There and Back Again is the title of Bilbo Baggins' memoirs, in which he records the events of the story that we know as The Hobbit. Tolkien wrote all his middle-earth stories with the conceit that he had found books from an ancient culture written in an ancient language, and translated them into our world's langauges -- the common language became English, and the other languages became invented languages which preserved the feel of the original from the perspective of someone who spoke the common language. For example, the language of the Elves sounded foreign, musical, and complex to the ears of the Hobbits, so Tolkien invented a language based on Finnish, which sounds that way to English-speakers.

The Hobbit is thus the contents of There and Back Again, with some explanation and framing story added by the "translator."