What does a vaccine contain?

Short answer: Dead and/or inactive pathogens. Explanation (also to a further question: "How does this prevent infection later?") : Pathogens are microorganisms that cause disease. When the body is exposed to dead or weakened forms of these microorganisms in a vaccine, your white blood cells are stimulated to create antibodies specific to that pathogen. If you are then exposed to the same pathogen, later on, your body will easily recognize it and be able to quickly produce a large number of antibodies again to fight off the infection.