lymph is a fluid in your body that consists of all of the stuff that exits your blood vessels at a tissue site, minus all the stuff that is reabsorbed. It has a lot of lipid in it, along with immunoglobulins and stuff like that. Lymph nodes are areas in the body where the lymph flows past which contain centers of the immune cells of the body. When a lymph node detects an infection in the body, they start to produce more immune cells and they grow, hence having large lymph nodes when you have an infection.