When a soul is not given the 'Soul Burial' by Zanpakuto soon enough, nature takes over. It may take hours or months, but sooner or later, the power that lurks dormant within the human soul begins to consume them from the inside. First, a large hole forms somewhere in the torso, representing the heart which has been devoured, as a black substance erupts from that hole in a swirling vortex, surrounding the figure in it's shroud. When the maelstrom subsides, a new creature is left in it's wake, it's face covered by a menacing white mask similar to the death masks worn in Latin American 'Day of the Dead' celebrations (which, in-fact, were done in honor of these 'gods' to whom Aztecan Spiritualist leaders and advisors offered sacrifices of budding Spiritualists, to consolidate their power and appease the Hollows with moderately strong souls.)
A hollow is a beast of instinct and hunger, though to what degree varies from hollow to hollow. In most cases, these creatures are consumed and driven by their hunger, and seek out whatever prey they can, beginning with those who were closest to them in life, and expanding out to ever yet stronger souls, frequently cannibalizing one another. At some point along the way, this ravenous need fades, and the Hollow ascends to something greater, evolving from a mere Hollow, to a Gruñido Menos, the weakest and most numerous of 'the few' as the Menos are called. Those Hollows who have overcome their instincts and regained their reason.
There are three ways the existence of a Hollow (including those who have ascended to the state of Menos or Arrancar) can end. The first is a proper soul burial, by way of a Shinigami's Zanpakuto, will purify the Hollow, and send it it and all the souls it has consumed to their respective afterlives as normal. Secondly, a hollow could be devoured by another hollow (including an Arrancar's Zanpakuto), in which case all composite souls become part of the devourer. Finally, a hollow can be destroyed outright, such as by a Quincy's arrow. When destroyed in this final method, all of the souls within the hollow (which may number in the thousands for a Gruñido Menos, or millions for a Vasto Lorde) are sent to the 'Waiting Realm', as though they had died in their afterlives. It is for this reason that the Quincy were purged for their Great Hollow Hunt, and the threat that such widespread Hollow killing posed to the continued life-cycle.