Hollow

Hollows are a race of creatures that are born from human souls that don't cross over to the Soul Society. After spending too long in the human world, these souls are corrupted and start devouring the souls of living and dead humans. These souls usually remain behind to guard a loved one or destroy an old enemy, although the target of their obsession will eventually move on. When they transform, this person is almost always the first soul devoured.

Hollows vary in appearence. However, they do share some similarities. All hollows where a white mask. Though the mask will vary in size, it is always shaped like a skull of some kind and serves the same purpose: to obscure the face of the original person. If the mask is broken, the original soul will be freed. This is temporary and the mask will eventually reform, reviving the Hollow. If a Shinigami cuts this mask in half, the soul will be purified of any sins it committed as a Hollow and either sent to the Soul Society or Hell, depending on the crimes it committed as a human. There is also a hole in the middle of their chest where their heart should be. The missing heart is actually the hollow's mask, which determines their powers and form.

Without their mask, hollows appear as they did before their transformation. The one difference between a hollow's soul and that of any other human is that hollow souls still have the hole in their chest where their heart should be.

Demi-Hollows
Earthbound spirits in the process of transforming into a hollow, they can only be seen when someone enters their territory. They are harder for Shinigami to detect then regular hollows.

Menos
Massive hollows created when multiple hollows, tired of human souls, come together and start feasting on their own kind. These are much more dangerous than regular hollows for obvious reasons. There are three types of Menos: the Gillian,which are massive spiked hollows with a spike-like nose coming from their mask; the Adjuchas, which are smarter but smaller than Gillian and prefer to fight by themselves instead of in groups, and the Vasto Lordes, which are roughly the same size as a human and more powerful than most Shinigami. Vasto Lordes are so powerful, it is believed that ten or more could destroy the Soul Society. Thankfully, only one has been encountered so far.

Arrancar
A hollow who has removed its own mask, gaining Shinigami-like powers.

Visored
The opposite of Arrancar, Visored are Shinigami who have aquired hollow powers, making them the only thing that can stand up against a Vasto Lorde. Unfortunately for them, this means they have been banished from the Soul Society.