Medium is at heart a blogging service, although right now it's only open to a select few authors. The posts are organized into "collections" that use different layouts depending on the content. One section, "The Writer's Room," appears to be organized by Emergence author Steven Johnson and is a plain text article template, while "Been There. Loved That." is a photo-driven page that looks squarely like Pinterest and is curated by a developer at the company.
The posts in each collection aren't organized chronologically. Instead, Medium uses a Reddit- and Digg-style upvoting feature that lets visitors rate each item, with the highest-rated content moving to the top of the heap to ensure better visibility. Currently anyone can sign in--using Twitter, of course--to upvote and leave feedback, but contributors are still on an invite-only basis.
Collections can be open to everyone, or closed to only a few authors. "Lots of services have successfully lowered the bar for sharing information, but there’s been less progress toward raising the quality of what’s produced," Williams writes in a blog post introducing the service. Yet he also says that Medium is built so that lots of people can easily contribute, and it's unclear what kind of controls the curator of each section has over what appears in their collection.
There is no mention of how Medium might make money, of course, and even the hint of advertising is noticeably absent from any of the pages.
Since Stone and Williams first worked together at Blogger, the fact that the most public product from their Obvious Corporation would be a blogging platform should have been, well, obvious.