What exactly is a soul, what defines it? This is the question I have attempted to answer.

We know the exact nature of a soul has never been uncovered as every mage who has attempted such a thing has seemingly vanished - the reasons for this is another discusion in itself - but we can speculate.
The first question to be answered is:
Is a soul simply the energy that powers a machine or system such as a body, creature, enchanted weapon or the Ingenium (http://www.uesp.net/wiki/Lore:The_Infernal_City/Items#Ingenium). Or is there more to it than that is the soul actually the manifestation of the conscious. Some sources cite tales of beings that could still remain in a state of semi-life ( The Underking for example (http://www.uesp.net/wiki/Lore:Zurin_Arctus)).
It would also seem that a soul is more like water - in the sense that small parts can be taken of it (http://www.imperial-library.info/content/oblivion-souls-black-and-white) - than say a block of stone.
On death unless trapped in someway by soulgem* or other means a soul is supposedly sent to the so called "dreamsleeve" to be "recycled". But souls themselves can interact and talk and in my opinion the soul is like half of a whole, the Ying where the body is the yang.

*Another issue I would like to raise is soulgems. What are they made of as they disentigrate after use and I cannot find any lore on how they are created.

The soul is a form of magic. upon death the soul is released to the dreamsleeve where it is recycled through nirn, and the process gives off magic. this is why Meridia (the Et' Ada of souls) split off from Magnus (the Et' Ada of magic) when Magnus decided not to take part in creation.

Yes but why can some souls communicate with mortals?

My view of the whole thing is : it seems that most ghosts/spectral forms are there because they have either been cursed or revived through necromancy.Curses probaly were put on them when they were alive,not allowing their soul to go onto the dreamsleeve.The soul,if it still plays part in an important event of some sort,can be allowed to communicate with mortals - those who are cursed but aren't really of any signifcance become the enemy ghosts we see in-game.

Necromancy is a more confusing topic,for me anyway.I view ghosts as a soul - and normal undead are simply animated beings.My theory is that souls take a while to get to the dreamsleeve so if a necromancer acts quick enough he is able to summon a ghost using the soul's body.

I think,once the soul enters the dreamsleeve and is "recycled" the soul can no longer go back to Nirn - yet the Dark elf racial sorts of goes against this as the ghost summoned is the elf's ancestor.Ways to explain this are :

1.Dunmer don't enter the dreamsleeve for some reason.
2.A "recycled" soul can somehow come back for a short while if his decendants know how to summon him.
3.Some other magika explanation.
4.Something else to do with the dunmer.
5.My idea is a load of balderdash.

The dead are nothing to be trifled with...

Anyway, maybe those who die have a choice to stay behind or move on to the Dreamsleeve to be recycled?

Wouldn't everyone pick that option?Being recycled doesn't sound very exciting,plus you can haunt all your enemies!

some souls are bound to Nirn by other mortals (soul gems or simialer objects) so they can be used in things like the dunmer ghostfences or so their magic can power animated bodies (skelotons or, maybe, Dwemer Centurions)

Other souls are bound because there is a task that they have left undone and their willpower is great enough to keep them here for now. Ie, the Forlorn Watchman.

Other souls are simply necromancic structures that are bound to Nirn to be servants. For example, Wraithes.

Finally, some souls are bound to a realm because the lord of that realm (read Daedra) wants them to be. Ie, Umaril.

If it helps anything, Nirn is a big ball of schizophrenia and the idea that identities are distinct atomic units doesn't quite apply.

AFAIK, though, soul gems only trap the "energy" of the soul released upon death, not the soul itself. I guess this is to avoid forcing the player to become totally evil just to stay in the game at higher levels.

I consider soul as some kind of energy construct. This "energy" is same for all, but uniqueness of "construct" determines uniqueness of soul, and, suchwise, personality.

Also I'm a bit sceptical about Dreamsleeve concept, especially about "recycling" part. Not in current Kalpa, at least, .

The only lore i know is the Imperial Census of Daedra Lords:
That Molag Bal is allowed his holiday at all hearkens back to a treaty of ancient times, when he reputedly lent his infernal power to the creation of the first soulgems.

In Daggerfall souls could break out from the enchanted weapons, and in Morrowind it was possible to identify which soul trapped in the gem, so some part of soul itself may be still survived in the soulgem.