Cancer is a group of more than 100 different diseases characterized by the uncontrolled, abnormal growth of cells. These cells form a lump or mass called a tumor. Some cancers, however, such as blood cancers, do not form tumors. Tumors can be benign (noncancerous) or malignant (cancerous). Benign tumors may grow, but they do not spread to other parts of the body and are usually not life threatening. Malignant tumors grow and invade other tissues in the body.

Sometimes cancer will spread to the lymph nodes. A lymph node is a tiny, bean-shaped organ that filters the flow of lymph, the clear fluid that plays a role in the body's immune system. Lymph nodes are located in clusters in different parts of the body, such as the neck, groin area, and under the arms. Cells from malignant tumors can also break away and travel to other parts of the body, where they can continue to grow. This process is called metastasis. Metastatic cancer is named for the part of the body where it started. For example, if breast cancer spreads to the lungs, it is called metastatic breast cancer, not lung cancer.

Cancer can begin almost anywhere in the body. Tumors are named for the type of cell where the cancer started. For example, carcinomas begin in the skin or tissue that covers the surface of internal organs and glands. Sarcomas begin in the connective tissue, such as muscle, fat, cartilage, or bone. Refer to individual Cancer Type sections for more information.