oEmbed is a format for allowing an embedded representation of a URL on third party sites. The simple API allows a website to display embedded content (such as photos or videos) when a user posts a link to that resource, without having to parse the resource directly.
I was totally unaware of this service, and the breadth and depth of it's capabilities, the possibilities are huge, I'm going to look at incorporating that into some of my pet projects as soon as I have a need.