According to Facebook, their Open Graph is an extension of their social graph and includes third-party websites, web pages that Facebook users have Liked, arbitrary actions and objects created by third-party applications.  These entities are then integrated with Facebook and its core functions.

Why do you need to know about Open Graph?

Open Graph is the structure beneath the entire Facebook network. It enables applications from third parties and maps out how each Facebook user connects with other people, Pages, places and things. Facebook uses the following image on their Open Graph page to illustrate how it works:

Facebook Open Graph