In a healthy body, natural systems control the creation, growth and death (called apoptosis) of cells. In this cycle of life, cells divide to make new tissue as older cells die.
When tissue is injured (say by a cut on the hand), the body's cell growth regulators react by speeding up cell division to make new tissue in the injured area as fast as possible. When the body has healed, the cell division goes back to the normal pace.
Cancer is a condition where the natural systems do not work right and cells do not die at the normal rate. As a result, there is more cell growth than cell death. Cancer cells divide without their normal control and make a mass of extra tissue.
As a tumor grows, it promotes the growth of new blood vessels (called angiogenesis) to bring in the oxygen and nutrients it needs. Cancer cells can leave the tumor site and travel through the blood stream and lymphatic system (the network connecting lymph nodes throughout the body) to other parts of the body. This process is called metastasis (meh-TAS-ta-sis). In the new site, the cancer cells may again begin to divide too quickly and make new tumors.