It is a community in which only adults and their children over 18 can live full time. It may have extensive recreational amenities as well as fitness and special interest groups for many activities. It almost always consists of one story homes with no more than 3 bedrooms. Adults under 55 may own under the HUD guidelines:
To be registered with HUD (The Federal Department of Housing and Urban Development) as an adult community 80% of the owners must be over 55 but 20% can be as young as 45. It excludes the community from the Fair Housing Law dealing with discrimination of families with children less than 18 years. They can legally prevent purchases by families who have children under 18.
http://www.hud.gov/offices/fheo/seniors/index.cfm