he human body is made up of many types of cells that are constantly dividing to produce new, younger cells that can “take over” for aging or damaged cells. Through this process, the body heals its injuries and keeps tissues healthy. Sometimes, this process gets out of control, and new cells continue to be produced even when they are not needed, forming a clump of extra tissue, a tumor.

There are two types of tumors:

malignant (ma-LIG-nant), or cancerous, tumors are made up of abnormally shaped cells that grow quickly, invade nearby healthy tissues, and often make their way into the bloodstream. When these cells travel to other parts of the body, they form additional tumors.
Benign (be-NINE) tumors are not cancer. They grow slowly and are self-contained; that is, they do not invade and destroy the tissue around them, nor do they spread to other parts of the body. Their cells are usually normally shaped.