The Imps were essentially fairies as perceived from a religious (and fearful) point-of-view. They were perceived as tiny demonic creatures, full of mischief. They were usually depicted as tricksters, and pranksters. Not major threats in themselves, but rather annoying. They were thought to seek out human companionship.
Imps were later cast in the role of witches' familiars, or even errand boys for their masters.