The Open Graph protocol enables any web page to become a rich object in a social graph. For instance, this is used on Facebook to allow any web page to have the same functionality as any other object on Facebook.

Twitter cards make it possible for you to attach media experiences to Tweets that link to your content. Simply add a few lines of markup to your webpage, and users who Tweet links to your content will have a “card” added to the Tweet that’s visible to all of their followers.

Meta tags are snippets of text that describe a page's content; the meta tags don't appear on the page itself, but only in the page's source code. Meta tags are essentially little content descriptors that help tell search engines what a web page is about.