You might have noticed an additional tab 'Communities' in the Google+ left sidebar. The Google+ introduced this after it hit the milestone of 235 million active users, a few 100k behind its rival Facebook. The Communities is basically a trump card of Google against Facebook Groups.
The new added feature allows users to create public or private communities on certain interests, topics, hobbies or organization. In simple words, the Google+ communities is just discussion forums within the Google+ networks.
The new feature has the capability to share with your community with any +1 button across the web.
Vic Gundotra- Senior Vice President, Engineering for Google said that "Today Google+ is the fastest growing network thingy ever. More than 500 million people have upgraded, 235 million are active across Google and 135 million are active in just the stream."
If you are interested in the Google+ communities and would like to create a community of your own visit https://plus.google.com/u/0/communities and click 'Create a Community' and then follow the Instructions.
You can also discover and join different communities by clicking the 'Join Community' Button at the top right of the respective Community page.
Read how Google+ Communities make Google+ for everyone at Why is Google+ Communities so great? by Mike. Here he mentions how he finds Google+ communities is better than you think.
What you think about the new Google+ communities, share your comments below.