I just came upon a site that talks about the various kinds/forms/branches of Necromancy. Now I do not know if this information is based on complete facts or not, but I guess that's also why im posting, so you(more expert) than I am can see for yourselves and leave any comments. I believe this is the most informative site on Necromancy i've found thus far.
here's the link:
http://www23.brinkster.com/falcis/spells.html