In old Germany, "elf" was a name applied to any kind of supernatural spirit, especially one that inhabited fields or forests. The Germans also blamed elves for sitting on their chests while they slept and causing them to have a nightmare.
In Scotland, England, and Scandinavia, "elf" was another name for a member of the fairy folk. Then, as fairy lore developed and became more intricate and complex, with levels and classes within their supernatural ranks, the English designated elves as smaller members of the fairy population and the Scots gave the title of elf to those beings who were generally of human size. Things changed a bit in Scandinavia, as well, when the people there began to distinguish two categories of elves— the benign light ones and the dastardly dark ones. Scottish lore developed to perceive the kindly elves as the "seelie court" and the nasty spirits as the "unseelie court."
Scandinavians also called the elves the "huldre folk" and envisioned two principle divisions of the beings. There were the lovely, charming elves, who easily passed for humans and who loved to join in folk dances and in village parties. These elves, especially the females of the bunch, could easily seduce any human male into obeying their will. The male elves, though appearing handsome and dashing in the firelight of a village festival, would usually be exposed as ugly when moonlight struck them in the forests. The Danes also noticed that even the attractive elves occasionally betrayed themselves with a long cowlike tail that popped out of their dress or trousers.
In contemporary presentations, elves are usually portrayed as jolly creatures, humanlike in appearance, but extremely diminutive in size, who love teasing humans and playing pranks on them.