What is it?
Think of Communities as Google's answer to Facebook or LinkedIn Groups. They are discussion streams centred around topics or common interests, aimed at helping users engage with others on the platform that might share similar passions.
You post in the same way you would elsewhere on the network (in fact you can cross post in Communities right from your main stream). You can also have public and private Communities - yet another step towards full-scale enterprise social networking.
As with many features that Google has 'copied' from other networks, there are a number of places where it has attempted to add improvements. For example you can filter streams by topics or categories.
And, of course, all the other Google+ features - from Hangouts and photos to events and +1s - are seamlessly integrated.
What does it mean for brands?
Well, if you've used Facebook or LinkedIn Groups then you'll probably see the potential pretty quickly. Google+ has already been embraced by many brands but often - as on other networks - a 'branded' page isn't the most natural place to reach out to other users. Common interest groups - structuring your discussions around themes - allows brands to talk in a less commercial way.
If you are an expert in a particular area, then Google is giving you another way to demonstrate this and connect with others that share this view. From a commercial standpoint, it's not hard to see how Google can tap into this interest graph to sell even more targeted ads. But there's also little doubt that this information will be used to optimise search as well.
Google has invested a lot in Google+. But the company hasn't just invested in the platform itself, it's invested in integrating it throughout its products and services. That's the true power of Google+ for those of us working in PR.
And Communities is just the latest part of the jigsaw.