In humans, the immune system consists of many different parts with many different types of cells. Broad categories of cells are the phagocytes and lymphocytes, whilst broad categories of chemicals are the cytokines, interleukins and the complement system.
The function of all these cells is to protect the body against foreign invaders such as bacteria, fungi, viruses and multicellular parasites. In addition it also protects the body against itself by destroying tumours (auto-toxicity).
Occasionally, the immune system overreacts and attacks normal cells. This results in an auto-immune disease.
Also see: white blood cell, antigen, antibody, vaccine, acquired immunity, autoimmune reaction, immunoglobulin, AIDS
For more than you probably need to know go read http://gsbs.utmb.edu/microbook/ch001a.htm