The immune system is the highly evolved set of mechanisms our bodies use to tackle any threats such as infections and cancers.
Immunology examines the structure and function of the immune system.
The British Society for Immunology have some great resources for basic explanations of immunology:
Immunology at the Cambridge Science Festival
Immunology in art
immuno-art is a campaign to support research at the Netherlands Cancer Institute to develop immunotherapies against cancer. It shows microscopic images of various tissues. Cells from the immune system are able to not only attack bacteria and viruses, but also to fight cancer cells.