Chronicle stablished that Death magic is something apart from fel or shadow. So, death magic is its own thing.
I think "necromancy" is a general term for a combination of forbidden magics that deal mostly with Shadow and Death.
Playable warlocks are more "practicioners of forbidden/dangerous magics" than fel-users. Affliction deals mostly with Shadow (maybe also Death), Demonologists summon things with fel magic (I think), and (Base) Destruction uses mostly raw power summoned directly from the elemental plane (thought you can use the green fire questline to actually use fel instead of elemental fire).
It should be noted that different magic forces can achieve similar goals. You can heal with Light, Life or even Fel. Same thing about raising corpses, we've seen it done with Death, Fel and Void, thought the later leave very corrupted undead (like the voidy Shadowmoon skeletons or that dragon raised by pitlord blood in Sunwell Plateau).