There are two primary forms of Necromancy described as True and Profane Necromancy. All magical experiments, spells, and acts that can be considered Necromantic fall under one of these two categories.
True Necromancy
True Necromancy is the actual summoning of the deceased. This can take many forms, the binding of a ghost to a new Ogen, reviving a recently dead corpse, returning the sentient being it once was to life. True Necromancy always requires a contract of some sort with one tether to the original Soul of the deceased party. Those who are granted divine magic are capable of forming this tether on the fly when confronted with death via the tether of their deity. In divine forms this contract almost always requires the consent of the Soul being summoned. However other, more diabolical methods exist in which Souls are tricked into making tethers and brought back to be questioned, trapped, or coerced.
Within True Necromancy lies one special sub-category labeled Auto-Necromancy. Involving the manipulation and experimentation on ones own Soul. It is extremely dangerous, but the potential power and longevity often get the better of many a wizard.
Profane Necromancy
Profane Necromancy, or pseudo-necromancy as many wizards have begun to call it has more general uses. The majority of spells and contracts deemed necromantic are really pseudo-necromancy, involving the dead, but no Souls. Simple animated dead of most kinds fall under pseudo-necromancies large umbrella. Usually powered by Monster Stones or Minor Stones of some kind, a contract provides them with a series of instructions to follow blindly. Every adventurer knows that if you smash the skull open on an animated skeleton you will find a Chip suspended in a gooey black membrane.