Hags are a kind of monster from Dungeons & Dragons. At their root, they are based in the stereotypical Western beliefs about witches - namely, that they are hideously ugly old women who use their dark magical skills to cause suffering amongst mortals for kicks -- mingled with assorted European mythology about ugly but powerful and magical female giants and trolls, from the Nordic Jarnvidjar to British boogeywomen like the river-dwelling child-drowning Jenny Greenteeth and the iron-clawed man-flaying Black Annis.
Because of this, they initially began as a kind of Giant, only to be redesigned in later editions (around third or fourth) as being a kind of fae embodying nature's capacity for cruelty and ugliness.
Hags come in many, many varieties, from water-dwelling sea hags to brutishly powerful annis hags to seductive green hags to the fiendish night hags and more besides. They all have a love for evil and specialize in using magic, most notably innate affinities for illusion spells and physically shapechanging, to interact with mortals undetected, when they don't just enslave armies of goblins, ogres, orcs and other evil humanoids and rule them openly.
As an all-female race, hags need to mate with humanoid men in order to propagate. In older editions, in fact, hags grow up believing themselves to be normal human women, only transforming into hags when they reach about their 40s or 50s. The except to these are Night Hags, at least according to Dragon Magazine, whose children are normal (if often cambions, due to the night hag propensity for breeding with fiends), but must be deliberately transformed into new night hags before they hit puberty. In the Pathfinder setting, hags give birth to more human-like daughters called Changelings, who they must "persuade" to be magically transformed into more of their own kind.