are a race of creatures which are born from Human souls who, for various reasons, do not cross over to Soul Society after their death and stay in the Human World for too long. They are corrupt spirits with supernatural powers that devour the souls of both living and deceased Humans. Hollows settle in Hueco Mundo, but can cross over to the Human World and Soul Society. Other species of Hollows are Adjuchas, gillian, Arrancar, Visoreds, and Soul Reaper-Hollow Hybrids. (tho Visoreds, Arrancar, and Soul Reaper-Hollow Hybrids are more Humanoid looking then the rest of the hollows and do not have thier instinct to kill and other insticts of Hollows; as they have more logical senses then Hollows)
An Arrancar is a Hollow that has removed its mask, thus gaining Shinigami-like powers. The word Arrancar comes from the Spanish verb meaning "to remove" or "to rip off".
The Visored are a group of Soul Reapers who have been banished from Soul Society after obtaining Hollow powers.
Soul Reaper-Hollow Hybrids is a new race of Hollow in DWS, that are far stronger then both Visored and Arrancar and are Soul Reapers who gained Hollow powers through illegal means and have become part Hollow and still remained as a Soul Reaper; so they are also part Soul Reaper. This race seems to be even more powerful to the vasto Lorde Hollow Class.