The lymphatic system consists of lymph vessels, lymph nodes and the lymph fluid itself. The lymph is not a closed system and is free flowing. The lymph fluid is formed when the blood plasma excretes out of the capillaries through capillary filtration. This plasma is roughly 20 liters, of which 17 liters is absorbed back into the blood vessels. The remaining 3 liters is known as lymph and the lymph system carries it so that it can be absorbed back into the main blood stream.
So the lymph fluid is actually the fluid running between the blood capillaries. It is actually the seeped blood plasma. The fluid is also known as the interstitial fluid that is present in the interstices of body tissues. Although the lymph fluid is plasma diffused from blood capillaries, it might differ slightly in composition. It has white blood cells and also lymphocytes.
The fluid is formed when the blood is at high pressure at the arterial side of the heart. Due to the pressure, some of the plasma diffuses into the environment around the capillaries since the vessels are insufficient to carry so much volume. The diffused liquid makes the lymph fluid which returns back to the capillaries at the venous side of the heart when pressure is low. All this transport works through the process of diffusion where plasma travels from the regions of high concentration to a region of lower concentration. Firstly, at the arterial side, the concentration is high inside capillaries. So it travels outside. When the concentration drops inside the capillaries at the venous sides, it enters back. The lymph vessels transport the lymph fluid to the lymph nodes. At the lymph nodes, any bacteria that is present in the lymph node or metastatic cancer cells are also destroyed. It eventually enters the circulation when it drains into either of the subclavian vein.