All types of cancer develop in our cells, the body's basic unit of life. To understand cancer, it is helpful to know how normal cells become cancerous.

The body is made up of many types of cells. Normally, cells grow, divide, and produce more cells as needed to keep the body healthy and functioning properly. Sometimes, however, the process goes astray -- cells keep dividing when new cells are not needed. The mass of extra cells forms a growth or tumor. Tumors can be benign or malignant.

Benign tumors are not cancer. They often can be removed and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body. Most important, benign tumors are rarely a threat to life.

Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control or order. These cancer cells can invade and destroy the tissue around them. Cancer cells can also break away from a malignant tumor and enter the bloodstream or lymphatic system (the tissues and organs that produce, store, and carry white blood cells that fight infection and other diseases). This process, called metastasis, is how cancer spreads from the original (primary) tumor to form new (secondary) tumors in other parts of the body.