That is a good question and also gave us our “Hot Topic” for the month of June. Here is how StumbleUpon.com describes themselves on their website:
StumbleUpon helps you discover and share great websites. As you click Stumble!, we deliver high-quality pages matched to your personal preferences. These pages have been explicitly recommended by your friends or one of over 15 million other websurfers with interests similar to you. Rating these sites you like automatically shares them with like-minded people – and helps you discover great sites your friends recommend.
StumbleUpon uses + or - ratings to form collaborative opinions on website quality. When you stumble, you will only see pages that friends and like-minded stumblers have recommended. This helps you discover great content you probably wouldn’t find using a search engine.
Searching vs. Stumbling:
Using search engines to locate relevant content typically means hunting through pages of results. Rather than searching for quality web sites, StumbleUpon members are taken directly to web sites matching their personal interests and preferences.