I think there is already a discussion of this over on the Lore section, but I am not sure.

Necromancy is the true form of magic and its limitless abilities. I say this because Necromancers tend to be some of the most powerful mages of all time wheter it is ES, DnD, Pathfinder, or whatever other RPG you would like to add they are almost always powerful mages. Necromancy is no different from casting a Fire spell, a Flesh spell to protect, or anything of that sort because Magic is the bending of energy and Necromancy bends the greatest form of energy, life. It gets a bad reputation for being evil, though it isn't inherited because the people who pratice it tend to become "mad" or "insane" when reality it is because they realize the truth of exictence. They soon realize Life is simply energy, has no real meaning, and it is no different than any other schools of magic. Though on the same side Necromancers become the way they are because they had a loved one who died too early or was killed as an innocent so they want to give them "life again". I mean Necromancy is not black and white because each person who practices it has a different story.

Mannimarco, the God of Worms, more or less became a Necromancer to defy a God, take over Bal's place as a Prince, and because he kept dwelling deeper in it and realized the true potential of it and how limitless it can be. Potemus, a character I created, became a Necromancer because he loves life and values it and after seeing a lot of innocent people die at the hands of the Stromcloaks and the Legion he realized he became a Necromancer. I know that seems like an Oxymoron if you value life and love life then why multiple it, well the answer is simple because he could fully give life back to those who died innocently whether it be a wife, husband, child, or whatever.

I know I am a Necromancer Apologist, and it always seems like I am constantly defending it, but that is because I have always played as one and know the different depths of one, and a Necromancer is just like any other class it can be Good, though not Lawful, Neutral, and even Evil. A Holy Paladin may seem good, what if they killed off an entire village simply because they believed in different Gods. Is that so different from an evil Necromancer killing for fun? What about an assassin who gets revenge on the person who killed his wife and child? Is that different from a Necromancer who wants revenge and wants to revive his family? The answer is No.

I could go on and on about this topic because I have had many discussions about it, but I doubt anyone wants to read five or so paragraphs of me defending Necromancy.

To answer, your second question a Skeleton is the simplest form of Necromancer and it is simply you giving life to bones, a Zombie is a step up because you are using more life energy to give form and are giving someone life, and a ghost is the hardest in a sense because you are manifesting the actual spirit of someone in a physical form. I hope that helps explained the difference if not, I will link you some books and such about the difference.

Necromancy: The Great Debate

I was going to link Karver's lore on the subject but he's done that. Well worth a read, and to me, still has room for ambiguity. The real dark art is the Necromancer's Moon stuff in my mind. Black Soul trapping is pretty grim, but even so you would be hard pressed to answer with 100% certainty that all black souls end up in the Cairn. The game implies it from Serana's dialogue, but also there is the possibility that only souls offered up go there rather than all souls. Whatever the case may be, while a sentient soul is trapped it is a pretty awful thing to experience and to do to anyone. The messing with dead bones and stuff could be argued that a Daedric soul is used to provide the energy, rather than ripping a mortal one from the sleeve. In that case, a necromancer animating the dead is not much worse than a conjurer besides the taboo that is using someone's grief so cruelly. An ethical necromancer may opt for donations like the resurrectionists of 19th century. It's an area where I am content to have ambiguity rather than be strict over.

I think for me to properly answer this, I need to have a better grasp of the lore surrounding death and stuff, but I'll give it a shot anyway. I don't think it's possible for there to be a right answer to this, it's such a complex question, with pros and cons that shift all the time with only one overarching question - do the ends justify the means? Of course, this changes from character to character, and I believe that scale has something to do with it. I'd say that ripping a soul from the afterlife (if thats what happens?) in order to save one life isn't justified, but to save 100? Yeah, the maths adds up to me. Then again there are more factors than that - who's soul are you stealing, who are the people being saved, is the solution short term or long term (ie is the result inevitable regardless).

Yeah I'm not sure man, I've confused myself already. Basically if the ends justify the means, yes, although the line seperating when it is and isn't justified is blurry at best.