Apoptosis is a type of cell death. It is also called programmed cell death because the cells follow a set of instructions which tell them how to die. These instructions allow the cell to die without damaging any of the healthy cells around it.
Over time, all healthy cells age and become damaged and some of these cells will undergo this type of cell death naturally. In other circumstances, the body uses this process to get rid of cells that are no longer needed.
A cell dying by apoptosis is called an apoptotic cell or apoptotic body.