Asphyxiation occurs when one's oxygen supply is cut off and air is unable to reach the lungs and surrounding blood supply; the brain will fail and the body will become unconscious within minutes without air. This also means that not only the brain is dead or beyond recovery, but also the organs of the body have been oxygen-deprived and have suffered damage as well. Death is imminent within minutes if there is no resucitation, if that's what you mean by outside influences.

Depends on the vector, choking on food doesn't do it like being hung from a rope until dead.

For more technical information and images and 'facts' try: http://www.scribd.com/doc/11437612/Asphyxia

http://www.dundee.ac.uk/forensicmedicine/notes/asphyxia.pdf makes for a decent read as well. This latter breaks it down into stages, along with breaking it down to causes and the effects of the cause.