Lymphoma is a type of cancer, an overgrowth of cells in the organisms. As it happens in other types of cancer, abnormal cells become capable of dividing rapidly and create an abnormal accumulation of aberrant and malfunctioning cells that keep dividing and changing in the process.
As they do, they would keep losing their normal restraints, become more aggressive, and start invading nearby and even distant tissues. However, lymphoma is a bit different from other types of cancer because it is not located in a single organ but rather in the lymphatic system as a whole.

Different from a single organ, the lymphatic system is a group of cells that run through the blood and lymph vessels and continuously circulates the body. There are various types of cells in the lymphatic system, and lymphoma affects the lymphocytes.
There are two main types of lymphomas, Hodgkin lymphoma and non-Hodgkin lymphoma. Moreover, there are plenty of subtypes in each category, and each one of them would have differences from each other. However, most types of lymphoma share almost the same basic symptoms. Read the early signs of lymphoma: