Open Graph is a protocol that allows any web page to become a rich object in a social graph. For example, this could mean that when you click on the Facebook "Like" button on a web page, your friend sees that you liked the page even if they're not on Facebook.
Open Graph is built on three core concepts:
1. Objects: A web page is an object, and can have any number of associated objects. For example, a web page about a band could have associated objects like an album, tour dates, and photos.
2. Properties: Each object has a set of properties that describe it. For example, the "name" property for a band is "San Francisco String Quartet". The photo for the band would have properties like height, width, and "author", which could be an email address or URL that represents the person who uploaded the photo.
3. Open Graph Protocol: It's a set of rules for how objects can define their own relationships to other objects. To get started, objects can declare which properties they have and what type of values they accept.
The Open Graph protocol allows for rich object descriptions that go beyond basic HTML and make it easy to publish richer data like: who wrote a book, the characters in a movie, or the events in a sporting match; or to control how we want our data to look when it's shared on social media.