An Elf is a mythical creature of various origins that is usually regarded as a good being that helps to make trees and nature good. It is therefore commonly associated with paganism and witchcraft (many modern witches today believe that elves are real creatures). Elves are often described as being invisible "unless you believe".
Elves are usually described as tiny, the difference between elves and fairies being primarily that elves cannot fly. In the Lord of the Rings, however, Elves were described as human sized (and if anything taller), hence changing the description of elves.
In most modern fantasy scenarios, Elves are one of the 3 main races (Elves, Dwarves and Humans), although if these 3 are described as the good races, they then become one of 6 races, including orcs, goblins and trolls and/or giants.
Elves usually have pale skin, are thin, and have pointy ears, with toes that point upwards. They are usually good with a bow and arrow and also very good with magic.
It is almost universally accepted in fantasy settings that the natural enemy of the elf is the dwarf.