Non-Hodgkin's lymphoma (NHL) is a form of cancer that occurs in the lymph nodes or other lymphoid tissues such as tonsils, thymus or gastrointestinal tract. These lymph nodes and lymphoid tissues make up the lymph system. 

The lymph system plays an important role in the body's fight against infection. It is made up of hundreds of small, bean-sized lymph nodes that are connected by lymph vessels. A clear fluid, called lymph, circulates throughout the system. Lymph is mostly made up of white blood cells called lymphocytes. White blood cells are important in the body's ability to fight disease, since they kill foreign matter in the body.

In NHL, some of the cells that make up the lymphatic system become abnormal. These abnormal cells divide rapidly, crowding out the normal cells. As the disease advances, it may spread to the bone marrow, central nervous system, liver, spleen and reproductive organs. The cause of NHL is unknown and may occur at any age from infancy through adulthood. In children, this disease is more common among 4- to 8-year-olds, and in teenagers. Non-Hodgkin's lymphoma is much different in children than in adults.