StumbleUpon chooses which new webpage to display based on the user's ratings of previous pages, ratings by his/her friends, and by the ratings of users with similar interests. i.e. it is a recommendation system which uses peer and social networking principles. Users can rate, or choose not to rate, any webpage with a thumbs up or thumbs down, and clicking the Stumble button resembles "channel-surfing" the web. Toolbar versions exist for Mozilla Firefox and Internet Explorer.
For those who enjoy my daily posts on this blog, add me as a friend on SU and stumble on more interesting stuff that makes me think!