Just What is a Fairy Door?

I first heard of fairy doors when I lived in Michigan. Ann Arbor is famous for its fairy doors. One can take tours (there are maps of sites), or the adventurous and persistent can Some people enjoy going on a fairy door tour; there are maps of fairy door sites; some folks like the thrill of the hunt.
A fairy door can be anywhere: on an inside wall, on an outside wall, Some are inside, some are outside. Some are high; some are low. The Ann Arbor doors are all "urban fairy doors." An urban fairy is a different species from your common everyday country bumpkin fairy. The urban fairy's doors look exactly like human doors, except smaller. According to the experts (self-proclaimed) an urban fairy is one-twelfth the size of a human. The doors are all locked, or that is, locked to humans. Would you want someone twelve times your size peering into your house? Whether the fairies have wee keys is not known. Some experts say a spell is suffiecient. Since there is no reported sighting of a fairy opening doors, the question remains unanswered.
Ordinary, that is non-urban fairies, live behind all kinds of doors. Some are round, some are rectangular; some look like stone or brick. Many fairies live inside trees, with their doors somewhere on the trunk.
Some humans make fairy gardens, complete with gates, fences, and plants.
Whether people want to attract the little beings or appease them is not clear. Fairies are not necessarily benevolent. Reports from Ireland are that fairies steal human children and leave fairy children in their places. Why? No one knows.