To understand the concept see the case study below.
“the Flickr slideshow of photos of the French employment riots [Flash required] amply demonstrates that, on Flickr at least, collaborative photojournalism is thriving. That success is at least as much a product of Flickr itself as it is a product of the contributing photographers. For those who don't know, Flickr allows members to upload photos to a public viewing area, and then "tag" them to denote their subject matter. Flickr then rates each photo according to "interestingness", a quality that is based on the ways in which other users interact with that photo. No one (outside of the Flickr team) knows exactly what that algorithm is based on, but I would guess that it measures things like the number of times each photo is viewed, the number of times another member calls it a favorite, the number of times it's emailed to others — those sorts of things.
The above slideshow consists of all public photos with a certain tag. So the first thing Flickr is doing is aggregating them. Then they are arranged by "interestingness" which means that the best photos (as judged by the community) come first. It also means that as new photos are added to the stream, it will continue to change, and more interesting photos will percolate to the top. If you haven't looked at a Flickr stream before, you'll be astonished by the high quality of these photographs.“
The Flicr show is viewable in Flash at http://www.flickr.com/photos/tags/cpe/interesting/show/