According to Advanced Pathophysiology, death starts a few minutes after death or when the brain dies. In here, there would be no pulse, no respiration and total pupillary dilation happens. That is why in movies, they usually check the person's eyes or pulse to confirm if he is dead or not. However, the heart dies first before the brain as science and medicine has described it in books. The brain dies approximately 5 minutes after the heart.
Immediate resuscitation is needed within 5 minutes to revive him or save his life. There are post mortem (after death) changes that the body undergoes. A few hours after death, the body starts to cool down or it is termed as ALGOR MORTIS. It is said that the body cools down for 1.5 degrees centigrade every hour.
After that, LIVOR MORTIS happens which is the bluish discoloration or cyanosis on the most dependent part of the body. When the circulation has stopped, the blood would stagnate, then due to gravity, it would accumulate at the most dependent part of the body like the back.
Then it would be followed by RIGOR MORTIS or hardening of the body. It happens in a gradual process beginning with the involuntary muscles, the smooth muscles and the last ones are the skeletal muscles.
That is why people who are dead would jerk and move, that is due to the skeletal muscles hardening. The last one is PUTREFACTION, or chemical change happens and yes, the yuckiest part of death, being eaten up by insects, decomposers and Pseudomonas.
Dead people turn greenish with many discolorations. The body release gas that smells really bad. Many would prefer the idea of a black hooded figure taking them or Nicolas Cage (City of Angels) fetching them rather than the idea of a million organisms eating every part of their body.
This would give a glimpse of what really happens after death of the human body. An estimated eight deaths for every 1,000 population and around 55.3 million die each year.