Fearabbit, your theory of conjuration seems to be most accurate, a 3 step process of sensing, teleporting and binding although i would say that the binding would come before teleportation for the sake of the conjurers safety, however this still leaves some questions. Why does the summoned creature stay for a temporary while and where does it go after it leaves, i just remembered that special spell in SI where you summon haskill, seem to recall that haskill reappears at his previous spot before summoning, although i am not sure if this would count as firstly it is a lesser power and secondly it is bestowed by the daedric prince of all things wrong and confusing. The only reasons which i can think of for a creature to return to its original position is that the teleportation is more of a 'borrowing' from that point in space to another. For the question of why they stay temporarily i believe that this can be safely attributed to magicka cost, while 'borrowing' the creature you have to pay an energy fee in the form of magicka, this fee firstly counts the creatures soul level and then the undefined value, this value can be as large as pleased and in the end the higher the value the longer it stays.
But there are still some puzzles i have been unable to solve, where do these creatures come from, and how is it that the specific creature you summoned is the one that came (e.g. you summon a ghost, a fellow conjurer summons another ghost, how does each ghost get selected to go to that conjurer?) Also this area where the creatures come from, do they have a depletable source of these creatures or is it infinite, can conflicts occur if a creature is sensed, borrowed and binded by more than one conjurer? (e.g. For sake of argument, if 10000000000000000000000 conjurers summoned a ghost at the exact same time will all the conjurers have a ghost? Would any conflicts occur because these ghosts had been double summoned, more than one connjurer?)
We haven't even touched upon the matter of black soul gems, such as their relation to necromancy, and people seem to have forgotten completely about my notes on schools and elements (check my first post in this topic if you don't know what i'm talking about), come on people show some enthusiasm and try to mention something from everything in your posts :wave:
(THIS S AN EDIT)
P.S.
I reckon summoning a skeleton stands in for just about everything described in Perparation of the Corpse. Which would be awfully tedious to do as a player and restrict every encounter with a necromancer to his lair..
That you can buy the summon skeleton spells in the guild of mages, well, blame gameplay.
i forgot to mention that i think that the summoning of skeletons spells is kind of the mages guild's way to lessen necromancy, one of the major cases on necromancy is that it causes people to commit inhuman atrocities like grave-robbing. When you sell spells that summon skeletons you satisfy some of the necromancers wants, having less people dabble in the dark arts.