A community would typically have a certain optimum size. That is, there would be enough people so that a diverse group of skills would cover what the community needs, and there would be few enough people so that every individual has a chance to influence the direction of the group.
Most likely that would mean that a community consists of between 10 and 200 individuals.
A community could be a group of people who live closely together, in a neighborhood, or even in communal living. Or, it could be a more geographically scattered group that is tied together by different criteria. Modern telecommunications can tie together a community of people that is scattered all over the globe.
In geographically oriented communities, living closely together, you will often find that the community aims at being self-sufficient. That is, it will take care of its own needs for food, energy, housing, education, etc. Usually not because the community wishes to be isolated. As a matter of fact you will find widespread networking and cooperation among communities. But rather, the community wishes to ensure that the needs are covered for all its members, and it is simply more efficient and satisfactory to cover most of those needs locally.
There can be a wide variety of living styles in different communities. A community is an excellent way for people of similar preferences to group together and cultivate their particular way of doing things. That might involve religious beliefs, sexual preferences, or cultural traditions.
Each community is free to set its own rules for how it operates. It is free to set criteria for membership of the community, rules for proper behavior, and rules for expulsion from the community for that matter. As long as the group minds its own business and doesn't try to impose those rules on other groups or individuals outside the community, that is perfectly fine. However, the community must present clear declarations of what the community is about. There would be no hidden rule one would be caught in by entering the community. Also, any member is free to leave the community if he or she no longer agrees with its mode of operation.
There will be varying degrees of how tightly a community is woven together. It might be a completely intentional community where everybody has chosen explicitly to come together and to share many activities and resources. It might also simply be the people in a neighborhood who by chance happened to live in the same place, and they might choose relatively few elements of their lives to share between each other.