Interface is defined as "a point at which independent systems or diverse groups interact". Technologically, it's "the point of interaction or communication between a computer and...[a] human operator".
For purposes of this post, we'll use a third definition: "a surface regarded as a common boundary", in this case the boundary between human beings and raw data.
A web browser reads data and reinterprets it as a visual experience. If you ever wondered what the web really looks like, go up in your browser where it says "File, Edit , View, etc.,", select "view" then select either "source" or "page source". MMMmmm.
So really good interfaces take that whirling mass of data and makes it visually compelling. Jonathan Harris's work, google earth and now Google Maps Streetviews, Twittervision 3D, KDDI "Eye" - here's three that take digg's raw material (stories posted by its users) and turned it into hypnotically addictive experiences. First check out digg. Then check out Digg stack, swarm(pictured below) and bigspy.
digg - stack
digg - swarm
digg - bigspy