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 usually described as a taller human but with more firmer touch to them. In the Lord of the Rings, however, elves were described as human sized (and if anything taller), hence changing the description of elves. They are sometimes known to have long, pointy ears.
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.