You can find some good info about it on wikipedia. But it really is something I would see a doctor about, it can be deadly if you crave to eat (and DO eat) some of the things I've heard of being craved.
It's not an eating disorder perse, but many pregnant women get it. It's when you eat things that are not considered food, and this includes ice. Ice is edible so I don't understand that, but it usually signifies a deficiency in certain nutrients, and foods.