Cancer occurs when cells in a part of the body begin to grow out of control. Normal cells divide and grow in an orderly way. Cancer cells do not. Cancer cells keep growing and crowd out normal cells. There are many kinds of cancer. But they all have this out-of-control cell growth in common.
Different kinds of cancer can act very differently. For example, lung cancer and breast cancer are very different diseases. They grow at different rates. They also respond to different treatments. Thatís why people with cancer need treatment that is aimed at their kind of cancer.
Even when cancer has spread to a new place in the body, it is still named after the part of the body where it started. For example, if prostate cancer spreads to the bones, it is still called prostate cancer. If breast cancer spreads to the lungs, it is still breast cancer. Sometimes cancer comes back in a person who seemed to be free of the disease after treatment. This is called a recurrence.
Sometimes cancer cells break away from a tumor and spread to other parts of the body through the blood or lymph system. They can settle in new places and form new tumors. When this happens, it is called metastasis (meh-tas-teh-sis). Cancer that has spread in this way is called metastatic cancer.
American Cancer Society website: http://www.cancer.org