Essentially, Stumbleupon sifts through the internet based upon the opinions of its users to separate what is worth noting from everything else. It uses a sort of trial-and-error method to determine what you like on the web, and what you would like to see more or less of. When you first register, Stumbleupon asks you to take a survey of sorts, and select individual categories or "tags" that you would enjoy finding new sites about. It takes this new information, and applies it each time you press the "Stumble" button on your toolbar.