A hag is a witch, or a witch-like old woman. Many fairy tales include a hag as a frightening character.
While the wide folklore and fairy tale tradition includes some hags with positive, benevolent qualities, you'll more often read about mean and ugly hags with evil intentions. The witch in "Hansel and Gretel" who tempts the children with her house made out of candy is one example of a famous literary hag. Mick Jagger of "The Rolling Stones" sang about another hag: "I was raised by a toothless, bearded hag." We doubt that, Mick.