A Necromancer is a kind of magic-user who practices necromancy(duh). "Necromancy" comes from the Greek words nekros ("dead body") and manteia ("prophecy"), and in its strictest sense, refers to the practice of communing with the spirits of the dead to learn about the future. Over the centuries since the coining of the word, its meaning has broadened to include any kind of magic relating to death and the undead, and thus the necromancer's portfolio has also broadened, in part due to confusion with nigromancy (literally "black magic"). Necromancers in more modern works are known to reanimate dead bodies, summon ghosts, and drain life-force from the living to fuel their ceremonies (or themselves). Surprisingly, they aren't generally known for fortune-telling nowadays.
Because death is scary, and in many cultures and religions, tinkering with life and death is reserved for the gods, necromancers are generally perceived as evil, and necromancy sometimes inherently so. A common storyline is that a person with magical talent falls in love, said beloved dies, the magic-user dabbles with necromancy to try to bring him or her back, loses sight of the goal, and before they know it, they're turning people into zombies and hamming it up like Skeletor. Some settings are shifting to a more nuanced approach that acknowledge that necromancy, like any skill, can be used for good or evil, but the majority of necromancers are Neutral Evil.
In settings where the spectrum of this school stretches pretty far, it generally tends to be that benign necromancy-related magic is referred to as "White Necromancy", with nasty undead-raising and life draining fuckery tending to be "Black Necromancy".
Dungeons & Dragons
Necromancy has been part of Dungeons & Dragons since the beginning. In one of the foundations for CoDzilla, wizards and sorcerers have been traditionally restricted to gray and black necromancy, whilst clerics have had access to white, gray and black, plus innate class features allowing them to manipulate/control undead. This is really frustrating for necromancer arcanists, who grizzle about evil clerics being able to do their jobs way better than they can.
Needless to say, D&D has made a lot of efforts to try and beef up the Necromancer, to varying levels of success.
1e and prior
In Advanced Dungeons & Dragons, the Complete Book of Necromancers introduced necromancy-specific kits, with perhaps the one kit that could dethrone a cleric for title of "the best necromancer" being the Undead Master, who could cast Enchantment spells and Command Undead and Extraplanar Beings like a cleric.
In BECMI, the splatbook GAZ3: Principles of Glantri introduced a new way of handling necromancers, as one of "the Seven Secret Crafts". These were not like the specialist wizard rules from AD&D, or the kits from there either. Instead, mechanically, they were closer to the prestige classes that would be invented in 3rd edition; once a Magic-User had attained a specific level, they could seek out entry in one of several specialist wizard orders. After spending time studying, which costs a certain amount of time and gold, and attaining a necessary level of experience points, an initiate gains access to a special spell-like ability they can use. Each of the seven secret crafts - Alchemy, Dracology, Elementalism, Illusionism, Necromancy, Cryptomancy, and Witchcraft - is divided into 5 Circles of power, each with its own unique spell-like ability which is harder to use and can be used less frequently. For example, the spell-like ability granted by reaching the 1st Circle can be 3 times per day, and chance of success is 60% + 1+ per magic user level, whilst the 5th Circle's ability can only be used 1 time per month and has a minuscule 20+1/level % chance of being used successfully. For necromancers, these powers consist of Protection from Undead, Control Undead, Create Undead, Raise Dead and Attain Lichdom.
3rd edition attempted to bolster the arcane necromancer through the use of prestige classes, like the "True Necromancer" (which required multiclassing as a wizard and a cleric) and the "Pale Master". Results were... kind of mixed. Especially since they did shit like create the Shroudmaster, a necromancer PrC specialized in controlling ghost-type undead... which only the Cleric can access. In regards truly fixing the necromancer, the closest they probably came was with a pair of alternate classes: a Necromancer class based on the Diablo II class, in their Blizzard-sponsored D20 game "Diablo II: Diablerie", and the Dread Necromancer, a Charisma-based spontaneous caster alternate class from Heroes of Horror.
In 4th edition, necromancers faded into the background, for much the same reason as Conjurers; WoTC struggled to find a way to handle their traditional focus on minions without unbalancing the game. Whilst conjuration would return in the first Arcane Power splatbook, necromancers were left out, as that book was only large enough to restore conjurers and illusionists to the 4e fold. Theoretically, an Arcane Power 2 might have brought back the necromancer in similar fashion, but 4e's cancellation to led to only two sources for official 4e necromancy.
Firstly, Dragon Magazine #372 featured the article "Secrets of the city Entombed", which provided necromancy-flavored spells for various Arcane classes - none of the traditional minion-mastery effects, but flavorful attack spells like Hungry Earth.
Secondly, In 4th edition, Necromancy appeared alongside Nethermancy as one of the possible Schools that can be selected by the Mage, an Essentials variant of the Wizard whose spells can thus be taken by real wizards as well.. Being an Essentials Necromancer was handled as a set of three features gained by choosing that specific magical school, and which were acquired at levels 1, 5 and 10. A Mage could also dabble in Necromancy by taking the 1st and 5th level Necromancy school benefits at levels 4 and 8.
- Necromancy Apprentice: When you hit at least one target with an arcane necromancy attack power, you gain 2 temporary hit points.
- Necromancy Expert: You gain a +2 bonus to Athletics checks and Endurance checks.
- Necromancy Master: Your arcane necromancy attack powers ignore necrotic resistance.
Necromancers made a full Player's Handbook return - and finally got a decent bone thrown their way - in Dungeons & Dragons 5th Edition. Indeed, they rejoiced with this edition, for they finally claimed their rightful place as the style's masters and kicked the cleric clean out of the ring for the right to claim that title. The edition even destroyed the concept of "Clerics of Death" being superior necromancer by redesigning their class features; the Death domain is focused on the "God of Death who kills people" archetype, with greater necrotic damage output, whilst the Graves domain is focused on the "God of Death who looks after the dead" archetype, and is thus specialized in fighting the undead rather than controlling them.
As for what necromancers got in 5th edition...
At level 2, their Grim Harvest ability lets them heal themselves by killing people (ie, it doesn't work on constructs or undead) with magic; they regain hitpoints equal to 2 (or 3, if it's necromantic) times the spell's level whenever they finish somebody off with a spell.
At level 6, Undead Thrall not only gives them Animate Dead for free, but also buffs their skill at using it; the amount of zombies & skeletons they can control with it is increased by +1, and the undead they create with this spell are tankier (+1 max HP per level of the necromancer) and fightier (add the necromancer's proficiency bonus to their weapon damage rolls).
At level 10, Inured to Undeath makes necromancers better suited to hanging around the undead, gaining Resistance to Necrotic Damage and immunity to effects that lower maximum hitpoints, both traits common to undead enemies.
Finally, at level 14, they gain the mother of all necromancer traits: Command Undead. This ability, the thing that traditionally shafted necromancers in favor of death priests, lets a necromancer attempt to enslave undead creatures that it can can see within 60 feet, though smarter ones have the ability to break free eventually, so be sure to cast Feeblemind if you want to keep them and don't care about them casting spells (for example, if you'd want a demilich minion).
Warhammer 40,000 features necromancy but not to the extent of Warhammer fantasy with the like of Nagash. We see in the history of Mortarion, Primarch of the Death Guard on the world Barbarus, a world wreathed in poisonous fog and ruled by necromancers who practised standard fantasy necromancy, and in the pen and paper role-play game Rogue Trader in the Koronus Bestiary, the BONE CONQUEROR a shard like creature that possess the dead, albeit standard necromancy is rarer but we do see a variant or something similar in the followers of Nurgle, the servants of Papa Nurgle can be like necromancers in many ways, especially when they start breaking out the Zombie Plague. Additionally, the way in which spiritseers are able to coax the souls of dead Eldar from the Infinity Circuits into wraithbone constructs technically makes them necromancers too, albeit of a different sort than the Nurglite kind mentioned above.
Warhammer Fantasy Battle's magic system features the Lore of Death as one of the eight standard schools of magic. Its practitioners tend to be a little gloomier than most, but the forces of Order are plenty willing to make use of them so long as they kill enemies. There are also regular necromancers. Long ago, in the land of
Egypt Khemri, there were a bunch of mage-priests who practiced a kind of magic in their rituals to honor the dead. But a mage-priest named Nagash fucked all that up, turning their entire country into undead, and trying to take over the world. Now there are two variants of necromancy, the uncorrupted kind practiced by the Khemrian Tomb Kings, and the regular kind used by necromancers and Vampire Counts.