Cancer cells are cells that have undergone a series of mutations that leave them able to divide continuously, according to About.com. Cancer is multifactorial, meaning that it takes several mutations for a cell to become cancerous, combining inherited and carcinogenic environmental factors.
Normal cells have biological checks and balances that signal them to die off when they become damaged or when they are no longer necessary, but cancer cells don't have this, notes About.com. The telomeres in normal cells break off and become smaller and smaller during normal cell division, causing the cells to die when they become short enough, but cancer cells can replicate their telomeres, achieving a kind of immortality.
Cancer cells also do not respond to biological signals (usually enzymes) when they reach a boundary, which is how they are able to continue growing into other tissues. Cancer cells lack adhesion molecules to keep them localized; this allows them to spread to other parts of the body, whether metastasizing into nearby tissue or traveling further to other parts of the body by way of the bloodstream and the lymphatic system, explains About.com.