This can be a very involved answer depending on what your specific needs and concerns are. But in general...
-amenities like pools, tennis, activities may be available
-rules & restrictions give an added level of protection from 'bad neighbors'
-community atmosphere, sense of neighborhood-ness
-dues to pay & the potential for assessments to pay for common improvements
-rules to follow & consequences for breaking them
-all communities are different & usually run by volunteer boards & paid management...level of 'strictness' can vary, as can level of services
Please let me know if you have specific concerns. I know communities pretty well, as I live in one, serve as President of a homeowner's association, and sell property & homes in many different ones. Also, check out my blog which may have some articles of interest on this subject.