Real-time updates are a powerful new tool introduced by the Graph API. Previously, due to restrictions in the way in which developers could store user data, applications using Facebook were forced to make numerous costly calls to Facebook servers to serve up data. With the introduction of the Graph API, though, Facebook changed the way it allowed data to be stored; now it may be stored, and you may subscribe to your users' data as it is changed over time.
In order to subscribe to real-time updates, you must register for the kinds of updates you want (presently, only User entities, as well as extended permissions and errors, may be tracked), implement a callback mechanism for the Graph API to notify you of changes, and then request those changes as you become aware of them.
For additional information, see the reference links below.