A diagram of the lymphoid system, courtesy The Emmir, via Wikipedia
The lymphoid system regulates the balance of fluids in the body as well as filtering those fluids to remove foreign objects and playing an important role in the immune system.
Lymphatic ducts stretch throughout the body and through osmotic action collect excess fluids from some parts of the body and deliver them to other parts of the body, including the bloodstream. Fluids travel through the ducts through peristaltic movement; muscular contraction squeezes the ducts and forces fluids through them. Like veins, lymphatic ducts have one way valves that ensure that the fluid cannot move in the wrong direction.
At certain points which are identical in all human bodies the ducts hook up with lymph nodes. The nodes contain filters which remove foreign material from the fluids before the fluids move on. The lymph nodes also have the ability to recognize foreign material, such as bacteria and viruses, and can trigger the production of antibodies.
Severe illness can block the lymphatic system and the nodes, leading to swelling of the nodes and edema in the affected area. One of the typical tests a doctor does during a physical examination is checking the lymph nodes near the surface of the body, which will be swollen if the patient is ill.