Google Plus is a social networking and identity service owned and operated by Google. Google Plus was launched in 2011 as an invitation only field test. A few short months later Google Plus opened to everyone without needing an invitation due to the high demand for invitations.
Circles is the core features of Google Plus, this allows users to organize people into groups or lists for easy sharing. Once a circle is created, the user can share specific photos or posts to that specific circle exclusively. This gives users more control on what content is shared and who sees it. For example, work related content can only be shared with coworkers or colleagues while personal content or photos can be shared with your friends and family only.