Open Graph Protocol

The open graph protocol is a standard for metadata to show previews of websites. Spearheaded by Facebook, it's an open standard that allows anyone to configure previews of images, videos, or other content from a website.

Here's the specification.

Twitter has their own approach, and Facebook has their own Facebook-specific previews as well, but the open protocol is what most websites use and/or will fall back to to display the site.