Cancer arises from a loss of normal growth control. In normal tissues, the rates of new cell growth and old cell death are kept in balance. In cancer, this balance is disrupted. This disruption can result from uncontrolled cell growth or loss of a cell's ability to undergo cell suicide by a process called apoptosis. Apoptosis, or "cell suicide," is the mechanism by which old or damaged cells normally self-destruct.
[This page was adapted from National Cancer Institute.]