Vaccination is the process of granting artificial active immunity to a disease.
Vaccination was discovered by Edward Jenner in 1796, when smallpox was a common and devastating disease. He noticed that people who got cowpox, a similar, but milder, disease, never caught smallpox. He tested this by injecting a boy with the pus from cowpox sores. The boy caught cowpox, but when exposed to smallpox, did not catch it. Jenner called this process vaccination from the latin "vacca", meaning cow.
Modern vaccination works by injecting a weakened or dead version of an pathogen. All pathogens have surface proteins or antigens, which can be used to distinguish them. In the bloodstream B-lymphocytes (white blood cells) have receptors which can bind on to some of these proteins. Different b-cells have different receptors, so many different proteins can be bound to. When a b-cell sucessfully binds to an antigen, it divides several times, into two different types of cell, plasma cells, which produce antibodies, which attack or hinder the antigen, and memory cells, which remain in the bloodstream in large numbers for a long time, this means that if the antigen returns the immune response is very much faster, as the correct b-cells are present throughout the body, and is the basis of immunity. In vaccination, the b-cells bind to the antigen, and the body gains immunity to the disease, but the pathogen is too weak to attack the body and overwhelm it.