Short answers, pulled from the story.
Bag End is the underground home of Bilbo and Frodo Baggins in J. R. R. Tolkien's fantasy novels The Hobbit and The Lord of the Rings. It is a luxurious smial, or Hobbit-burrow, dug into the Hill on the north side of Hobbiton in the Shire's Westfarthing. Both Bilbo and Frodo set out from Bag End on their adventures and return there.
"Bag End" was the real name of a Tudor home dated to 1413 in the village of Dormston, Worcestershire, owned by Tolkien's aunt Jane Neave. The scholars Steven Woodward and Kostis Kourelis have also suggested that Iceland's turf houses, such as those at Keldur, may have influenced the design of Hobbit-holes.
Tolkien scholar Tom Shippey identifies "Bag End" as a direct translation of the French cul-de-sac, meaning "bottom of a bag," used in England to mean a dead end or road with only one outlet. In both The Hobbit and The Lord of the Rings, the name mirrors the story's structure: Bag End forms one end of the main story arc, and the Hobbits always return there, forming what scholars call a story circle.
The cartographer Karen Wynn Fonstad drew a floor plan of Bag End based on clues in Tolkien's novels, making it some 130 feet long and up to 50 feet wide, cut into the Hill, with multiple fireplaces, chimneys, cellars, and pantries. Thomas Honegger writes that Fonstad's work contributed substantially to giving Middle-earth an "independent existence."
Peter Jackson built the Hobbiton film set, including Bag End, on the Alexander sheep farm at Matamata, New Zealand, for his The Lord of the Rings film series. The set included a water-mill, the Green Dragon Inn, several Hobbit-holes, and Bag End itself in a small hill with a garden.
Tolkien scholar Johanna Brooke, writing in the Journal of Tolkien Research, identifies Bag End as an Arts and Crafts building, fitting the ideas of designer William Morris and his circle between 1880 and 1920. Its panelled walls, tiles, carpet, and round windows could all, Brooke argues, have been manufactured by Morris & Co., marking the home's owner as prosperous middle-class.