oEmbed was introduced recently (beginning of this month) as a standardized way to get enough info about a resource to embed it; ie, get the format, dimensions, url, title, etc., so that you can build an <img> or <embed> tag for rendering the resource on your page. This is an extremely useful idea; it would let people paste a URL places rather than the youtube embed codes or whatever. However! This scheme requires two URLs to represent the same resource; a more RESTful approach would simply ask the resource's URL for a particular HTTP response. Backdrifter explains: oEmbed FAIL! Represent RESTfully.
(similar, but less readable spec: XRDS)