A vaccine is a substance designed to trigger the immune system into launching an attack against invading viruses, before a person becomes ill. For example, the flu vaccine contains pieces of the flu virus; the human immune system will detect this as an invasion and begin producing extra immune cells to fight infection. Cancer vaccines are similar in that they incorporate tumor cells to trigger the immune system.
But cancer vaccines are different in one very important respect: they are designed not to prevent cancer, but to treat those who already have it, usually in combination with other therapies.
For some reason, the mutations found in cancer cells are tolerated by the immune system, which is not triggered to attack them; tumor cells have developed a way of evading detection by blending in with healthy cells. This is why we need cancer vaccines -- so the immune system gets help identifying and seeking out cancer cells.
There are many different types of cancer vaccines being developed: some are designed to treat specific types of cancer, while others are universal vaccines, which fight cancer of any type. For example, some carry proteins from a tumor cell; by injecting these into the cancerous area, the immune system will send extra killer T-cells to that area to attack those types of cancer cells.
The human immune system is a network of immune cells produced in the bone marrow from stem cells. The immune cells circulate through the body in the blood, or are stored in the lymph nodes located at various spots in the body. Some immune cells are more general, patrolling the body and clearing away dead cells, viruses and bacteria. Other cells are activated only by a single substance (called an antigen), such as a particular protein on the surface of the virus. These are called T-cells. When T-cells detect the presence of an antigen, they multiply to combat the invading virus.