What Is It?
Cancer is not a single disease. It's a group of diseases characterized by their ability to cause cells to change abnormally and grow out of control.
Most types of cancer form a tumor, a lump or mass of cancerous cells. Cells from a tumor may break away and travel to other parts of the body, where they can settle and multiply. This spreading process is called metastasis. New cancers that have broken off and spread from the original tumor are called metastases.