Not restricted to one species, refers to a vocation a sage, magician or most commonly a Sorcerer can take.
As long as there has been life, death and the abilty to control it, there have been necromancers. However, for eons it was (in in some parts remains) a immoral practice.
Refers to the ability to perform magic’s concerning the dead etc raising the dead, controlling the dead and reposing spirits into the dead. The line of work also involves taking spirits from corpse, but this is technically called called exorcism and in some realms the two occupations are exclusive. The abilities and power levels vary from necromancer to necromancer.
Different processes of necromancy vary legally from world to world (it is considered immoral in ours, for example), and mostly it is strictly governed and necromancers must be licensed and watched, much in the same way as a doctor (indeed, necromancers names are preceded by Nec, pronounced Neck-sir, like Dr)