Cancer cells will often travel to other parts of the body where they begin to divide, grow, and replace normal tissue cells. This is called metastasis. Cancer travels to the different body areas through the bloodstream or lymph vessels.

Cancer cells develop when a person's DNA becomes damaged. It is normal for DNA to become damaged, but most of the time the damaged DNA is easily repairable by the body. In cancer cells however, the damaged DNA is not repaired, but remains damaged. When people inherit cancer it is through this damaged DNA. Many cancers are not inherited, but are caused when a person’s DNA becomes damaged by exposure to a substance in the environment, such as the smoke from cigarettes.

Cancer usually forms as a tumor. A tumor is a collection, or group of abnormal cells. One common cancer that does not form tumors is leukemia. Leukemia cancer cells involve the blood and blood-forming organs (such as bone marrow), and circulate through other tissues where they grow.

Not all tumors are cancerous. Benign (noncancerous) tumors do not spread to other parts of the body and are rarely life-threatening.