I had a little trouble understanding Circles in Google Plus myself, so I created this visual guide on how the Asymmetric Sharing model works:
Google Plus has a different privacy/sharing model than other social networks. Different from Facebook's confirmed tie model that they have been gradually breaking with ever more granular complexity. Different from Twitter & SlideShare's model of Asymmetric Follow, where people can follow/subscribe to posts that are public. Google+ is Asymmetric Sharing, where you selectively choose to share, but to make it into the main stream of view, someone nees to choose to share back. I give it high marks on privacy, but it inherently comes at a cost for complexity, virality, discovery and retaining social context.