Normal body cells grow, divide, and die in an orderly fashion. But cancer cells continue to grow and divide, they are different from normal cells. Instead of dying, they outlive normal cells and continue to form new abnormal cells.

Damage to DNA is the main cause for the development of cancer cells. This substance can be found in every cell and it directs all activities. Most of the time when DNA is damaged the body is able to restore it but in cancer cells, the damaged DNA is not repaired. People can inherit damaged DNA, which accounts for inherited cancers. DNA is mostly damaged by exposure to something in the environment, like smoking.

Cancer usually forms as a tumor though some cancers, like leukemia, do not form tumors. Instead, these cancer cells involve the blood and blood-forming organs and circulate through other tissues where they grow. Not all tumors are cancerous. Benign (no cancerous) tumors do not spread (metastasize) to other parts of the body and, with very rare exceptions, are not life threatening.

Cancer cells often travel to other parts of the body where they begin to grow and replace normal tissue. This process is called metastasis. Despite the consequences of where a cancer may spread, it is always named for the place it began. For instance, breast cancer that spreads to the liver is still called breast cancer, not liver cancer.

Different types of cancer can act very differently. For example, lung cancer and breast cancer are very different diseases. They grow at different rates and respond to different treatments. That is why people with cancer need treatment that is aimed at their particular kind of cancer.