| Gather 'round the campfire|
“”Up the airy mountain,
Down the rushing glen,
We dare not go a-huntingFor fear of little men.
|—The Fairies, William Allingham.|
Fairies (or Faeries, if you
want to be pretentious/edgy like to spell it like that) are mythological creatures commonly found in European folklore. What we would know as fairies evolved from English folklore with very heavy Celtic, Roman, German, and Scandinavian influences.
There are a variety of Irish legends of the Fair Folk, or aos sí. Despite the fact that these creatures were reputed to be mischevious, if not actively malevolent, they were called "fair" or "good." To do otherwise was seen as an easy way to arouse their anger. Despite the Catholic Church frequently labeling these beings as fallen angels or even daemons  many of the Irish still, to an extent, revered them. Behaviors related to this reverence were known as the Creideamh Sí, or "fairy faith."  Such practices included leaving an offering of food such as milk, baked goods, and berries, and continues in the present, usually in the form of avoiding construction near sites considered important to the Fair Folk. 
Many of these beliefs (or close variations thereof) are shared by the Scots.
“”You see the elf-king, fahter? He's near! The king of the elves with crown and train!
|—Erlkönig, by Johann Wolfgang von Goethe (1782), translated by Edwin Zeydel (1955)|
In countries with a traditionally Germanic mythological basis, such as Germany and Scandinavia, the equivalents are known as elves. Even after the Christianization of Northern Europe was completed belief in elves continued parallel to other religious practices, similar to their Celtic counterparts.
Elves were often seen as a source of illness, the "elf-shot" one notable example. Martin Luther, for instance, believed his mother was sick due to the influence of elves, reflecting a continued pagan tradition in a notably devout Christian. Indeed, where elves were once seen (relatively) neutrally, the Christianization of Germany painted them in a more negative light. Again, this is not dissimilar to the Celtic Fair Folk.
Elves (more specifically, their artistic depictions) were a cause of competition during the Romantic period of literature and art, with various nations trying to lay claim to being the true "progenitor" of elves.
While they are mostly relegated to fantasy literature today, there remains some trace of belief in these creatures. Iceland is notable for its greater continuity to Old Norse traditions due to its relative isolation, and belief in elves (or even an unwillingness to express disbelief) continues to this day. A startlingly large proportion of Icelanders, in a study by Professor Terry Gunnell of the University of Iceland, believe that it is at least possible for elves to exist. 
“”The series of incidents set forth in this little volume represent either the most elaborate hoax ever played upon the public, or else they constitute an event in human history which may in the future appear to have been epoch-making in its character.
|—Sir Arthur Conan Doyle, falling for not the most elaborate hoax|
The first person to examine the series of five photos taken by Elsie Wright and Frances Griffiths outside the family was Edward Gardner, with the help of photography expert Harold Snelling. Gardner notes in a letter to Doyle "... two [other expert photographers] did claim that they could produce the same class of negative by studio work...", but these objections were "were disposed of almost at once" by testimonial of the girls and by Snelling's examination.
- Lewis, C. S. (1994 (reprint)) The Discarded File: An Introduction to Medieval and Renaissance Literature. Cambridge, Cambridge University Press. p. 135-6 ISBN 0-521-47735-2.
- It was thought by some that fairies were fallen angels who weren't quite evil enough for Hell, but still had to pay tithe to Satan.
- Evans Wentz, W. Y. (1966, 1990) The Fairy-Faith in Celtic Countries. Gerrards Cross, Colin Smythe Humanities Press ISBN 0-901072-51-6
- The Coming of Fairies (1922), p. 13
- http://archive.org/stream/comingoffairies00doylrich?ui=embed#page/n57/mode/2up The Coming of Fairies], page 48f.