Cancer is the uncontrolled growth of abnormal cells in the body. Cancerous cells are also called malignant cells.
Cells are the building blocks of living things. Cancer grows out of normal cells in the body. Normal cells multiply when the body needs them, and die when the body doesn't. Cancer appears to occur when the growth of cells in the body is out of control and cells divide too rapidly. It can also occur when cells "forget" how to die.
There are many different kinds of cancers. Cancer can develop in almost any organ or tissue, such as the lung, colon, breast, skin, bones, or nerve tissue.
MedlinePlus - Cancer