There are many vaccinations for different diseases. However, when getting chickens from a hatchery, many will choose to have them vaccinated for Marek's disease and/or coccidiosis as day-old chicks. Mareks is not effective if given after that age, but you can buy it online to give to chicks hatched at home as day-olds. Some of the hatchery websites can educate you on what each vaccine is for. Here is one site that sells Mareks and others: http://www.jefferspet.com/vaccines/camid/liv/cc/3501/
There are many different vaccines for several diseases that can be given to chickens. The one disease that chickens should be vaccinated for is Marek's Disease (most hatcheries do this for a minor cost). Other common vaccines include those for Infectious Bronchitis, Coccidiosis, Newcastle Disease, Fowl Pox, and Infectious Laryangotracheitis.