Immunisation is what happens in your body after you have had a vaccination. The vaccine, which contains inactivated or weakened viruses or bacteria (or parts of them), stimulates your immune system so that it can recognise and protect you from future infection (i.e. you become immune to the infection).
Vaccination and immunisation are often used interchangeably but their meanings are not exactly the same.
Read more in-depth information about vaccines and immunisation.