A mass of cells growing out of control is called a tumor.

There are two main types of tumors:

* Benign tumors: These tumors are non-cancerous. They grow slowly and do not spread to other parts of the body. They can often be removed surgically.

* Malignant tumors (cancers): These tumors are cancerous. They grow quickly and can spread to other parts of the body (metastasize). They are often more difficult to treat.