Dogs are social animals. This behavior allows them to establish an order of dominance, to cooperate in pack pursuits like hunting, and to just enjoy being together. Pet owners have found that they can increase the quality of life of their dogs by using these instincts in controlled environments like dog parks. However, there are also risks involved, both to the dogs and the owners, when dogs are brought together. The AVMA has published an excellent group of articles on minimizing the chances of bad things happening in controlled interactive environments.  

The link is: http://www.avma.org/animal_health/disease_risks_dogs/default.asp . A printable version is also available at the site.