Sometimes known as Genies or Gremlins. Originally they were the offspring of the devil, however the puritans often associated their name with any evil spirit. Today we see Imps as little devils with pitchforks.
Historically the Imp was thought to be small Genie kept in a bottle or ring (see Ifrit). When it was waken, it would serve itís master with Magic. There are supposedly good and bad Imps.
During the mass witch hunts of the Middle Ages Imps became associated with familiars and were interchangeably used. It was said that Witches used Imps to do deeds against innocent people, in return being able to suck the blood from a witches finger. Hunterís used this theory to find witches by looking for sores, teats etc on peoples bodies to indicate being a witch.