There are three types of necromancy, that which involves the spirits of the dead, that which involves the places where the dead go, and that which involves the flesh that they leave behind. Which are you looking for information on?
well, that's a long and lengthy subject. Spirits either stay bound to earth usually due to trauma or because the spirit feels it needs to accomplish something.
Where they go, is like saying where do people go, well there is China, and America, oh and Europe, etc. etc. there is not just one place, and many factors dictate the fate of those who die