Cancer is a term used for diseases in which abnormal cells divide without control and are able to invade other tissues. There are several main types of cancer including carcinoma which begins in the skin or other tissues, sarcoma which begins in bones or muscles, and leukemia which starts in blood-forming bone marrow and causes abnormal blood cells. Lymphoma, myeloma, and central nervous system cancers are other types that begin in immune system and brain/spinal cord cells, respectively.