Google+ (read as Google Plus or abbreviated to G+) is a social networking service owned by Google launched late June 2011.
It's the search engine company's attempt to create a rival to Facebook. Google+ integrates Google's other social services including Google Profiles and Google Buzz.
One major concept of Google+ are circles. Much like groups or friends lists in Facebook, circles allow users to categorise their contacts. These contacts themselves do not know which circle they have been placed in.
Content sharing between Google+ users is asymmetric and best described in the below flowchart: