Within this post I hope to share what I have learnt about this subject. It is quite hard to find information on. All if the information I have found is from various sources. Ars Falcis, Various People and a few books have led me to learn quite a lot about the subject. But I lack experience so forgive me that I can not speak from what I have done.
So then, Necromancy is normally known as Divination through the dead. This is only partially correct. Yes it is true that the name means Dead Divination in Ancient Greek but it is more about understanding death and it's energies. Many of the meditations and practices there help you to feel what death is like. It also connects you to what necromancers call the "Netherworld".
The Netherworld is were the dead go, some say it is part of the Astral and some say that it is it's own plane (I can't say I agree with either, I have never been there). I have even heard it mentioned that it is part of our plane. Just not visible to a untrained eye. When a Necromancer has visited the Netherworld a fair few times it is said that he can become more open to it's energies. Such as sensing when things from there are about. It is also said that sometimes you can leave part of your spirit there. So it may learn and experience things while you are physically awake, this is said to ruin the body though (such as more prone to disease, always tired and weak).
Now the main thing that most people wonder the most about is Zombies. Now from what I know this is a teem used for a servant kind of thing. They are on the Netherworld and/or the Astral (depending in if you believe the astral and netherworld are the same). They even help out on our plane. It is supposed that through meditation and certain "mind tricks" you can turn a human into a mindless servant. This may have been were the myth came from. A Necromancer works quite closely with emotions so understands how they work (well kinda). This can allow him to "play" with a persons mind and make them feel things that aren't their. Basically like a psychologist (I am studying various parts of psychology and have done for a very long time). I'm not sure if this is a true thing or not though, I don't know anyone that has tried it.
Now finally in this post I would like to tell you about what spirits and entities are connected to necromancy. There is no set of god and goddess for this practice/life style. But it is worth looking up Archangel Azreal (no he is not a demon! He is a archangel and always has been). Azreal was tasked with studying death and it's energies, something no other angel would do, after he did this he was often called the angel of death and second in command of hell. But this is a lie, made by the Christian church. But it is true that he wears a black robe (and I'm not sure about the scythe, will look into it). Also there is something known as the Graveyard Guardian (that Flagg mentioned to me) now I'm not 100% sure what he/she can do. All I can gather is that he/she guards the dead and their graves (as suggested in the title).
So thanks for reading
Be Well and Love, Learn and Grow