Cancer develops when cells in a part of the body begin to grow out of control (see pic).  Although there are many kinds of cancer, they all start because of out-of-control growth of abnormal cells. Normal body cells grow, divide, and die in an orderly fashion.  Instead of dying, cancer cells continue to grow and divide. They outlive normal cells and continue to form new abnormal cells.

Why do cancer cells develop?  Because of damage to DNA. DNA is present in every cell and directs all activities. Most of the time when DNA becomes damaged the body is able to repair it. Not so in cancer cells. People can inherit damaged DNA, which accounts for inherited cancers. More often, though, a person’s DNA becomes damaged by exposure to something in the environment, like smoking. See picture here.

Some good information is from the American Cancer Society, ASCO, NCI, and Medline plus.