By joining a community, you can see updates being shared by other users in that community on your Profile wall.
If you do not want to see certain updates from certain communities, you can filter the updates that appear on your Homepage by using the filter tool.
In order to post in the forum for a particular community, you need to have joined that community.
You can join one or many communities and can leave a community at any time.
In order to leave a community, visit the Communities page where you will see all the communities that you are a member of.
Go to the forum for the community that you wish to leave.
Click the 'Unjoin Community' button. You are no longer a member of this community.
You can re-join a community at any time.