Apoptosis is a "planned" cell death, where either the body commands the cell to kill itself because it's no longer useful or the cell is mortally wounded and dies a tidy death by suicide for the good of the body.

In contrast, cell necrosis is sudden, "messy" cell death that is not considered apoptosis. It can cause problems for surrounding cells as debris, signaling molecules, and even digestive enzymes get released uncontrolled from the dying cell.

Cancer cells are damaged (usually genetically) in a way that causes them to ignore the body's signals to commit apoptosis, but also to ignore signals to not divide and to stay where they are.