Lymphoma is a cancer of a part of the immune system called the lymphatic system. There are many types of lymphoma. One type is called Hodgkin's disease. The rest are called non-Hodgkin's lymphoma.
Non-Hodgkin's lymphomas begin when a type of white blood cell, called a T cell or B cell, becomes abnormal. The cell divides again and again, making more and more abnormal cells. These abnormal cells can spread to almost any other part of the body. Most of the time, doctors can't determine why a person gets non-Hodgkin's lymphoma.
MedlinePlus - Lymphoma