"Humans, when they die, their soul-- a ghost, if you will, is left. Such a spirit is called a "plus". Pluses wait until they are taken to Soul Society by a Reaper."
Ulquiorra paused, not quite sure why he was explaining this to the other man, he supposed he was just bored.
"However, if a plus is left waiting too long, they eventually start to hunger and want for life, for attention, for anything... If left alone, they turn into a hollow. A hollow is a spirit who has lost its human heart. Hollows devour pluses in order to rid themselves of the emptiness they feel. Eventually, seeking power, a hollow starts devouring other hollows and evolve and regain some consciousness. An Arrancar is something of a more advanced Hollow. We have gained the power of a Hollow, as well as some of the powers of a Reaper."