What is Graph API?

A Graph API is an interface that enables applications to read and write data to and from a data store that is modeled as a graph. The most well known example is the Facebook Graph API, which allows developers to query data about users, pages, groups, events, and other objects on Facebook, as well as post new stories, upload photos, manage ads, and perform a wide variety of other tasks. It's commonly used to integrate applications with social networks or any service which exposes its data as a graph.