Hags are female magical beings with somewhat evil tendencies, similar to humans but less adept than witches at disguising themselves from Muggles.
Hags are wild in appearance, and they have been known to eat children (FB, DP, FW), although at the Leaky Cauldron Harry once saw a hag eating raw liver (PA4). Hags can be quite frightening, apparently, since Quirrell had a "nasty bit of business" with one during his year off for first-hand experience in fighting the Dark Arts (PS5). Interested parties may also consult Gilderoy Lockhart's book Holidays with Hags (CS4), which probably can be picked up quite cheaply at Flourish and Blotts these days.
Known hags include:
The word "hag" has referred to a witch or a repulsive old woman since the 13th century. The origins of the word are obscure, but it is connected to the word for "hedge." A witch was considered to be a "hedge-rider" where the hedge was seen as the dividing line between the cultivated, civilized world and the wilderness beyond. A hag, then, is someone who lives in both realities.
There was no masculine form of the word, and all the witches in fairy tales are depicted as women. Rowling's hags also seem to be exclusively female.
Hags are the wizarding world's equivalent of "fairy tale witches."