In the middle ages fairies were synonymous with wild women, i.e. women of the wood, the water and the natural world in general. Norwegian mythology tells us that larvae, which came out of the giant Ymir's body turned into light elves and dark elves, who live underground, are dark-skinned and evil with malign influences. The Irish version says that Eve was bathing her children on the riverbank when God spoke to her. She was afraid and dismayed so she hid the children she hadn't washed yet. God asked her if all her children were there. She replied that they were. Then God said that those she had hidden from him would be hidden from men too. These hidden children became elves and fairies and in Scandinavia they were known as the people of Huldre.
Elsewhere it is believed that fairies are fallen angels or dead pagans, not good enough for heaven but not bad enough for hell, forced to live halfway between the two forever, in the twilight zone, or the Middle Kingdom. 

Fairy tale iconography portrays them as weightless, winged figures but there are actually infinite varieties of fairies, even nasty, deformed ones. They are always ready to rush to the aid of persecuted innocents. But the general models of fairy behaviour vary considerably: some are definitely bad by nature and are made even more dangerous because of their beautiful appearance that they use to ensnare unsuspecting men who then meet with a horrible death. 
According to some traditions they assist at the birth of humans to give them special gifts and influence their lives in a good or bad way. They are primitive beings that do not have feelings and so they are envious of humans. Like gnomes and pixies they guard great riches too.
Musical fairies and elves possess a magical ability and play a wide variety of instruments such as the violin, the harp, the cymbals and the Jew's harp. Many ballads and songs all over the world originate in the Fairy Kingdom, including the Londonderry tune. They often dance on the grass in rounds known as ‘fairy rings'. We know that fairy melodies are beautiful and haunting as well as wild and unpredictable, and that they have a fatal attraction that can draw passers-by into the circle, which, like fairy kisses, food and drinks, can reduce them to eternal slaves in the fairy world.