Open Graph protocol

Open Graph (OG) is Facebook's protocol allowing interactions between the elements of a web app and Facebook platform. The most common use case for OG is Facebook displaying an image and short teaser text attached to an URL you share on the Facebook platfom. The content of the elements added to the URL are defined by tags within your website's header.

Read more here: http://ogp.me/

Clearing og cache

 * Instructions on how to clear Facebook cache for og:image and other things