Summary:

Ghosts are the consciences and memories of dead people/animals, supported by “spiritual” energy.

Reproduction:

When someone's heart stops, the soul leaves the body, still chained to it by some sort of umbilical cord (this chain is also present in a creature without a bellybutton). This chain disappears when the brain starts to sustain too much damage to work even with a re-established blood and oxygen flow. And so, the soul becomes a ghost.

If the chain is broken by a third party, like a reaper, for example, the brain will automatically die. (not really sure why it does that, and none of the experts I managed to talk to seem to know for certain either, but some think it does this to avoid the creation of a second soul)

Biology:

They’re made of spiritual energy and always appear with pale cyan coloration. Besides that, their looks and form will depend on how the creature saw themselves before they died.

Normally ghosts of animals are just shapes without much detail, and the ghosts of bling people or people that couldn’t remember what they looked like will also appear as silhouettes. Some creatures die metalized of the mortal and gruesome wound that killed them, and so these will appear on their bodies when they become a ghost. But all of this can be fixed when they reach the afterlife.

Additional facts:

Some ghosts can choose to trade their place in the afterlife by the power to influence the world around them. AKA: They gain the power to possess people, meddle with objects, etc.

Spiritual energy is finite, and the amount of energy a ghost has depends on their body mass when they were alive. If the energy depletes completely, the ghost will disappear forever.

Because of this, some ghosts that traded away their place in the afterlife, and ghosts who want to avoid punishment, end up practicing cannibalism and slowly become an amalgam of consciences with an insatiable hunger, a Soul Eater. Some of these monsters can even become powerful enough to kill the living.

Personal experience:

I don’t have any, but one of the soul tamers that I interviewed assured me that they are, and I quote: “A bunch of entitled blue clouds that will drain your sanity”.