So, in essence, necromancy was (traditionally) a divinatory art (in short asking questions of the dead who were seen to stand outside time [the notions of time and mortality being intertwined]). Greek and Roman Necromancy by Ogden is a good text on the history here.

Nigromancy, translating to "black divination" (black as in dark/evil magic) was more to do with demonic pacts and spells focused around power (e.g. summoning a feast, calling up a castle, cursing to madness or death, winning the love of a woman). The text forbidden rites gives a good exploration of this from a historical perspective.