Immunisation is any process that provides an individual with immunity to a pathogen. This may be achieved through natural infection or through vaccination. It can also be achieved by giving the individual immunoglobulin containing antibodies to the pathogen.
Active immunisation may give individuals "immune memory", meaning that their immune system is "primed", and able rapidly to generate antibodies if the same antigens are encountered in the future; or it may generate antibodies without generating immune memory. Many simple polysaccharide vaccines do not generate immune memory (and may only generate humoral immunity. In contrast, most live organism vaccines and conjugate vaccines generally do generate immune memory, and sometimes also cell-mediated immunity.
In addition to providing immunity to the precise antigens contained in the vaccine, many vaccines also provide cross reactivity and thereby cross protection against related antigens and pathogens.