Start by watching the Crash Course video, Lymphatic System. 

The lymphatic system collects fluid that leaks out of the capillaries. The lymph vessels have valves to prevent backflow from occurring. The lymph nodes filter the lymph (excess fluid) and destroy foreign matter. The lymph vessels then return cleaned fluid to the circulatory system. However, if the lymph vessels or nodes are damaged, then lymph collects in an area. 

Lymphocytes are a subtype of white blood cells found in a vertebrate's immune system.