Whats it like to be hung, drowned, decapitated or burned alive? This interesting, if gory, article addressess the more common ways of dying.
Death comes in many guises, but one way or another it is usually a lack of oxygen to the brain that delivers the coup de grāce.
Whether as a result of a heart attack, drowning or suffocation, for example, people ultimately die because their neurons are deprived of oxygen,
leading to cessation of electrical activity in the brain - the modern definition of biological death.