Facebook Graph API is a structured API for fetching objects and the connections between them from Facebook's social graph
The Facebook Graph API is provided by the Facebook platform to interact with Facebook's social graph. It's a low-level HTTP-based API that you can use to query data, post new stories, manage ads, upload photos and a variety of other tasks that an app might need to do.
This includes Facebook users, pages, and other objects, as well as the connections between them such as links, photos, or notes.
It provides a structured view of the Facebook "social graph" and represents objects in the graph (e.g. people, photos, events, pages) and the connections between those objects (e.g. likes, uploaded, is tagged in).
Tool Graph API explorer - The easiest way to understand the Graph API is to use it with the Graph API Explorer, a low-level tool you can use to query, add and remove data. It's a very handy resource to have at your fingertips while you integrate with Facebook.
You can ask questions with tag which requires Facebook Graph API calls for above objects only with different SDK's.
Resources