Fairies are a humanoid species on magical realms who can change their size from human to a very small size, with insect-like wings. They are native to Fairy Tale Land, Wonderland, Wish Realm, New Fairy Tale Land, and many other magical realms.
Biology
Fairies are human in appearance, but with a few noticeable differences. Normally, they are quite small in size, although they have the ability to enlarge themselves to the size of a human. Two dragonfly-like wings protrude from their back, which disappear when they are not flying, and re-appear as they get ready for take-off. When they fly or change shape, their bodies emit a sparkling glow. ("Dreamy", "Quite a Common Fairy".)
Only female fairies have appeared so far, and it is not known whether there are any male fairies in existence. However, unlike dwarves, fairies are capable of falling in love and have been known to enter into relationships with humans. ("Dreamy", "Trust Me".)
Fairies have their own special form of magic called Fairy Magic. They all wield magic wands, and their power is fuelled by Fairy Dust or Pixie Dust, which is manufactured by Dwarves. If a fairy strays too far from their chosen path, their leader may stop believing in them, and they will lose their wings, along with their powers, and will no longer be a fairy. However, it is possible for a fairy to regain their powers if their leader and the fairy herself starts believing in her again. A fairy can also give up her wings voluntarily, after which she will no longer be a fairy. A human who is versed in magic can transform herself into a fairy by using another fairy's wand and a special spell. ("Dreamy", "Quite a Common Fairy", "Going Home, "Awake", "The Black Fairy".)
There is an unknown, likely large number of fairies, as seen when a horde of them attacks King George's castle. ("An Apple Red as Blood".)
Fairies appear to be a long-lived species. During the time where Rumplestiltskin (who was more than a hundred years old by the time the Dark Curse was cast) became the Dark One, the Blue Fairy was already a legend in the Enchanted Forest and was described as an ancient being. The Black Fairy, who is Rumplestiltskin's mother, still looked young when she was reunited with her son more than a hundred years after she gave birth to him. ("The Return", "Changelings".)
Culture
Fairies generally spend little time outside their own realm. They go through a training program with other, more experienced fairies, until they are able to work on their own. Fairies dedicate themselves to helping others and doing good. The more experienced fairies often become protectors of children, families, and other people in need, and become their "fairy godmothers". This job is not limited to one person or family. ("The Shepherd", "The Price of Gold", "Dreamy", "Quite a Common Fairy", "Changelings, "The Black Fairy".)
Fairies have to respect some rules, including a curfew, dust discipline and not taking on human size for no reason. If a fairy stops being good, she may be exiled. ("Quite a Common Fairy", "Going Home", "Changelings".)
According to the Blue Fairy, fairies don't lie. However, the Blue Fairy has lied on at least two occasions: She told Snow White and Prince Charming, at Geppetto's insistence, that the magic tree could only protect one person from the Dark Curse, when, in actuality, it could save two; and when Rumplestiltskin asked the Blue Fairy if a curse can be used to travel to the Land Without Magic, she says no, even though she witnesses the creation of the Dark Curse in person. ("The Return", "The Stranger", "The Black Fairy".)
Fairies have their own written language, which some humans can read as well. ("Changelings".)
Fairies are closely related to stars. The Blue Fairy can be summoned by wishing upon a blue star and is also known by the name Reul Ghorm, which means "blue star" in Scots Gaelic and is also the real name of Mother Superior, the Blue Fairy's Storybrooke counterpart. The Black Fairy can similarly be summoned by an incantation that calls for the "dark star" to fall. While other fairies have not been given corresponding stars, Nova's name refers to "supernovas", an exploding star. ("Dreamy", "The Return", "Darkness on the Edge of Town", "Changelings".)