A Hag is a supernatural being found in many mythologies and folklore - although many Hags are neutral rather than evil they have become heavily associated with witchcraft and evil, to the point the two are often interchangeable (much like how Ogres and Giants have become somewhat merged in modern times).
Traditionally a Hag was seen as a spirit or goddess in the form of a hideous old woman and often had power over the elements or magic - thus gained fear and respect in equal lengths by the superstitious, who would try to either appease the more benevolent of these spirits or ward off the more malicious.
Hags were likely inspired by pagan beliefs dating back almost as far as human history and probably embody part of the ancient concept of a "Goddess" - as Christianity spread however Hags would inevitably become more and more demonized (along with many of the "fairies"): giving rise to tales of grisly old demon-witches with a taste for human flesh and so on.
Famous hags of a villainious nature in folklore are the Russian Baba Yaga, the Orcadian Scota Bess and the English Black Annis.