Immunisation is the process of granting immunity from something to something. This is most often used to refer to vaccination, where immunity to a disease is artificially granted by an injection of a weakened or dead strain of an antigen, or by an injection of antibodies. It can also be used legally to refer to immunity to prosecution and occasionally in other contexts e.g. immunity to certain weapons (in RPGs).
Medical immunisation can be either active or passive, and artificial or natural. Active immunity occurs when b-lymphocytes bind to an antigen, different lymphocytes have different binding sites, only some of which will allow the lymphocyte to bind to the antigen. The lymphocytes that bind will then divide, so many cells with the correct binding sites are produced, some of these, called plasma cells, produce antibodies that can bind to the antigen, others, called memory cells, stay in the bloodstream in case the antigen returns, enabling a much faster response if the antigen returns.
Passive immunity is where antibodies from another source enter the body, conferring tempory immunity to the specific disease.
Artificial immunity is man made, such as vaccinations, or injections of antibodies. Natural immunity is produced by the body, such as when an antigen enters it, or antibodies given to a baby in its mother's milk.