You can be brain dead or heart dead, or both. Brain dead means that the body is still functioning, that the heart is pumping and the blood is circulating, but there is no el…ectric activity left in the brain. In this case, you usually also say that "the person has left the body", you no longer have a personality, thoughts or emotions. If you are heart dead, it means that the heart is no longer pumping blood, and the lungs cannot suply your body with oxygene. This will lead to brain death as well. Well, the answer to this can vary to many different subjects, all too big to explain..
The main way for someone to die is for their heart to stop functioning and their body to slowly change into a state of death. One can be brought up after experiencing this, but only for a short amount of time will this opportunity stand. After that the body becomes useless, and in religious cases, "empty." The "soul" of a person is meant to either go into a state of purgatory or Heaven itself..
Some may die of diseases..
The main requirement for life is Oxygen, without oxygen, cells and tissues, and eventually organs will die and stop functioning. Therefore anything which stops the oxygen supply around the body will cause death. There are many causes of this including a clot (thrombus or embolus) in major arteries, brain damage which affects heart function and respiration, and disease, amongst many others. ( Full Answer )