Routine use of appropriate vaccines is an integral part of preventing infectious disease in your horse.
A horse's need for any particular vaccine depends on the degree of risk that it is at of getting that disease. For some diseases, the risk is high (e.g. tetanus) whereas for other diseases, the risk may depend on a number of factors such as how frequently it associates with other horses or the geographic area that it lives in.
The American Association of Equine Practitioners divides vaccines into "core" and "non-core" (Ref.: www.aaep.org.) Core vaccines are those that meet several criteria: are endemic, are of potential public health significance, are very infectious and associated with risk of severe disease, or are required by law. Core diseases include tetanus, rabies, Eastern Equine Encephalomyelitis (EEE), Western/WEE (Venezuelan/VEE in some areas of the world) and West Nile.
Non-core vaccines include influenza, rhinopneumonitis, and strangles. For these non-core vaccines, you and your veterinarian will work together to decide how much risk your horse is at for these diseases, and plan an appropriate vaccination schedule accordingly.
Certain other groups of horses may have special requirements in regards to vaccination. Pregnant and breeding mares, foals and weanlings, and the older or geriatric horse are examples.