Communities make it possible for people with similar interests to get together
Google is always improving Google+, the latest addition are Communities, its take on Facebook Groups. It's not a direct replacement for Groups, rather Google wants it to gather people with common interests and expertise.Anyone can create a new community and they can even mark it as private so that only the people they want can join.
Given that Google+ already has great sharing controls, via the Circles, Communities are an extension of that. Apart from sharing something with a circle, you can also do it with a community now.
The feature is also similar to the forums of old, as they allow different conversations or topics to be contained within a community.
There's really nothing new in concept, Google Groups has been around for ages for example. But the Google+ integration means that the feature has a much better chance of picking up steam and communities a much better chance of getting members.
Communities can be entirely public, meaning anyone can join, or public but with membership required to post something. Alternatively, there can also be private communities that can be discoverable or completely hidden to non-members.
The feature is on its way, but Google hasn't made the announcement just yet, for some reason. Once it's live, you'll find it at plus.google.com/communities.