While there's no official Potterverse description of a hag in Fantastic Beasts and Where to Find Them, in Chamber of Secrets, Harry describes Millicent Bullstrode of Slytherin as looking like a hag he saw a picture of in Gilderoy Lockhart's book Holidays with Hags. Presumably, a hag can be female, large in stature, with a square-shaped body, and sporting a heavy, jutting jaw. A female hag may be larger than a male human. Note: I don't know if in Chamber of Secrets Harry was still a bit small and skinny for his age, so take my last statement with a grain of salt.
Malfoy strutted over, smirking. Behind him walked a Slytherin girl who reminded Harry of a picture he’d seen in Holidays with Hags. She was large and square and her heavy jaw jutted aggressively. Hermione gave her a weak smile which she did not return.
Chamber of Secrets - page 143 - Bloomsbury - chapter 11, The Duelling Club<
Hermione and Millicent Bulstrode were still moving; Millicent had Hermione in a headlock and Hermione was whimpering in pain. Both their wands lay forgotten on the floor. Harry leapt forward and pulled Millicent off. It was difficult; she was a lot bigger than he was
Chamber of Secrets - page 144 - Bloomsbury - chapter 11, The Duelling Club
In Prisoner of Azkaban, Harry encounters what he believes to be a hag having a breakfast of raw liver in the Leaky Cauldron. If this is indeed a hag it would seem that hags in Potterverse do assimilate into wizarding society, at least enough to appear in public at the Leaky Cauldron and eat raw liver.
Harry ate breakfast each morning in the Leaky Cauldron, where he liked watching the other guests: once, what looked suspiciously like a hag, who ordered a plate of raw liver from behind a thick woollen balaclava.
Prisoner of Azkaban - page 42 - Bloomsbury - chapter 4, The Leaky Cauldron
Apparently Hogsmeade is enticing to magical beings, it being the only fully magical town in Britain. Harry thinks he sees another hag in the Three Broomsticks and notes that hags are not as adept as disguising themselves as wizards are.
The Three Broomsticks was packed, mainly with Hogwarts students enjoying their free afternoon, but also with a variety of magical people Harry rarely saw anywhere else. Harry supposed that as Hogsmeade was the only all-wizard village in Britain, it was a bit of a haven for creatures like hags, who were not as adept as wizards at disguising themselves.
Goblet of Fire - page 280 - Bloomsbury - chapter 19, The Hungarian Horntail
If I find additional information on the hag, I will edit it into this answer.
ETA: Yes, I realize that it says Millicent reminded Harry of 'a picture he saw in Holiday with Hags' and does not specifically say 'a picture he saw of a hag in Holiday with Hags'. However, as the book is titled Holiday with Hags, I don't think it's much of a stretch to imagine the picture Harry referred to seeing in the book was quite likely a hag.