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. 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. 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.