A user may click on a Circle on the left side of the page and the Stream portion of the page (the center) will contain only posts shared by users in that Circle.
For the unsegmented Stream (includes content from all of a user's Circles), each Circle has a "slider" configuration item with four positions: nothing, some things, most things, and everything.
Users were originally able to create Shared Circles, a one-time share to promote a circle of individuals, but the feature has been removed.
Another function of Circles is to control the content of one's Stream.
Ron Arvine, President of Arvine Pipe & Supply Co., Inc.