But the general idea is to cause an immune response without bringing about the actual illness.
Sometimes it is an inactivated or altered form of the virus.
Sometimes it is a more benign strain (the word vaccine comes from the latin for cow because the small pox vaccine was cow pox and not small pox) that can cause the body to make antibodies to protect you against the more virulent strain.
Sometimes it is a dose of the actual virus that is low enough such that it causes the immune response but not enough to make you sick.