Ghosts

Those who wander, lost.

Forgotten souls

Ghost are the souls that were left behind.
In normal circumstances, souls will be collected by a reaper upon death, but in cases where a soul leaves the body through other means, either willingly or unwillingly, and if they are unable to return to their body, they eventually become ghosts.
The souls that were left behind. ghosts live fully in the the spectral realm, meaning they can see all, but they  are invisible to those in the physical realm. The ability for a ghost to interact with the physical world entirely depends on their magical ability, a person with an average magical ability, will barely be able to lift a feather that is in the physical world.
People with the sight are able to see into the spectral realm, and therefore see ghosts, they are often mistaken for living humans from a distance, but up close, their transparency and slight blue-ish hue are unmistakable.

Fate

Ghosts can often be brought back into the world of the living, either by helping them get back into their original body, or, if that body is no longer alive, a mage can call upon a reaper, to come collect them, and bring them to the land of the dead. Often these lost souls are taken by Ynue, and abused for their vile purposes, this is not something the soul often survives, and sadly, they are lost forever, corrupted, or simply destroyed, depending on the circomstances

Related Articles:
The spectral realm 
Reaper