Cells either die via necrosis or apoptosis. Apoptosis is programmed (to maintain normalcy within an organism), whereas necrosis is due to external factors such as injury or poison.
Apoptosis is controlled so that unneeded or potentially dangerous cells are eliminated. During apoptosis, the cell commits suicide. The organelles, DNA, and eventually the entire cell splits into small fragments. These parts are devoured by the phagocytes/macrophages.
Apoptosis is critical to maintaining homeostasis (internal stability) and getting rid of cancerous cells. Unlike necrosis, it is beneficial.