The initial version of DhO left many of the senior contributors wishing that there was a place for a more focused and specific vision of how web dharma should be discussed and transmitted. Moderators sometimes had widely different ideas about what was acceptable and proper. Communities are a way to allow groups that wish to do some their own thing to have a place to do that.
Most are simply collections of people who have a common interest and are open to all. Others are closed except to members, as that's the way that group wants it, and who are we to say? At least they can draw on the greater resources of this site, and hopefully will engage with the wider group also. Each Community has its own blog and pages of its resources. Those that are private have their own discussion sections.
Please join as many Communities as you wish. However, realize that each has its own rules, moderators and ideas about what is appropriate and accepted there, and to please accept them on their own terms and behave when you post and participate there.
Use the window to the right to find the Communities. Once you have joined one, you can find it under "My Places" in the Welcome drop down menu at the top right.
Should you have a vision for a Community and the interest and the time and commitment to moderate it, you can discuss starting one with Daniel Ingram.