Communities has yet to roll out to most people. It will be an additional icon on the left column of Google+, presumably leading to a directory of existing Communities to join. Users will also be able to start new Communities and regulate membership.
Google+ Communities are intended to be a way for like-minded people to organize or interact around topics of common interest or need. Here are the rules and contours of Google+ Communities:
- Public or private membership to support all kinds of groups—from topics and interests to local neighborhoods to regular poker nights
- Discussion categories to find the conversations you care about most
- The option to start hangouts and plan events with community members
- The ability to share with your community from any +1 button across the web
Yahoo Groups has equally been used by local-school parents and adult services vendors. Google will need to make decisions about whether Communities will be open to all types of groups and content (e.g., porn, radical Islamists) or whether it will impose content restrictions.
For related information and more context, see Danny’s discussion of the Google+ usage numbers released today. The video below (effectively a commercial) provides some sense of how Google+ Communities operates.