Coma's what are they? what kind different stages are there?

A coma is a deep state of unconsciousness, during which an individual is not able to react to their environment. Someone in a coma cannot respond to any form of stimulation whatsoever. Coma can be caused by an underlying illness, or it can result from head trauma. A comatose person is still very much alive, but he or she is not simply asleep. The brain wave activity in a comatose person is very different from that of a sleeping person; you can wake up a sleeping person, you can’t wake a person in a coma. A coma usually does not last for more than a few weeks. Many people recover their full physical and mental functioning when they emerge from a coma. Others require various forms of therapy to recover as much functioning as possible. Some patients never recover anything but very basic body functions.

Sometimes, following a coma, a person may enter what is known as a persistent vegetative state; patients in persistent vegetative state have lost all cognitive neurological function but are still able to breathe and may exhibit various spontaneous movements. They may even be awake and appear to be normal but, because the cognitive part of their brain no longer functions, they are not able to respond to their environment. A vegetative state can last for years.

There are various other terms, in addition to coma and vegetative state, that are used to describe varying levels of unconsciousness and a person’s ability to respond to stimuli. These include stupor, in which a person is unconscious but will eventually respond to repeated, vigorous stimulation; and obtundation and lethargy, which are used to describe a person who is not entirely unconscious but does not respond to stimuli.

http://www.neurologychannel.com/coma/