A retirement community, or active adult community, is a very broad, generic term that covers many varieties of housing for retirees and seniors - especially designed or geared for people who no longer work, or restricted to those over a certain age . It differs from a retirement home which is a single building or small complex where no "common areas" for socializing exist. Many retirement communities are planned for that purpose, and have special facilities catering to the needs and wants of retirees, including extensive amenities like clubhouses, swimming pools, arts and crafts, boating, trails, golf courses, active adult retail and on-site medical facilities. Other facilities have no or very few common amenities. An Age-restricted community generally requires at least one household resident to be 55 plus years of age or older (occasionally 50+ or 60+ years of age). Retirement communities are often built in warm climates, and are common in Arizona, California, Florida and Texas but are increasingly being built in and around major cities throughout the United States.
Naturally Occurring Retirement Communities (NORCs) can involve low-income residents receiving a richer mix of public services through a NORC model. They might serve people of all income levels who got together to furnish cost-effective transportation services. And there are NORCs for relatively affluent households that may charge $1,000 or even more in annual dues, and support paid and volunteer staffers who provide a rich variety of support services and cultural enrichment activities. NORCs can be very effective mechanisms to identify populations of people who need government-provided services and then provide those services in cost-effective ways. An example would be Beacon Hill Village in Boston  provide support and resources for those who do not want to leave their current neighborhoods. Another term may be used for a predominantly senior citizen community, in which residence is unrestricted by age and job affiliation.
There are really three broad categories of retirement communities:
ACTIVE communities (all residential units, no long-term healthcare facilities - also known as "independent living communities" such as Holiday Retirement which has over three hundred communities across the US.
ACTIVE/SUPPORTIVE communities (a combination of residential and healthcare facilities - also known as "continuing care retirement communities" - CCRC)
SUPPORTIVE communities (all longterm healthcare units, like assisted living facilities or nursing homes)
A number of publishers have created lists of the 100 best retirement communities or "100 best places (or towns) to retire" . For the most part these lists are helpful in terms of finding desirable communities to live in. One drawback to these lists is that these communities often become more expensive as a result of their popularity