The principle of StumbleUpon, is that every user can add interests. Then the user can start to "stumble" and based on the interest a random page of that topic will appear. It's like a random site generator based on your interests. So users can add links to websites too, and when they do they have to add topics related to this website.

The user also can like or dislike pages. The more likes a website has more likely it will be shown to users.

So StumbleUpon is like a randomized website finder based on your interests. If your bored next time then go and "stumble".