StumbleUpon is a web browser plugin that allows its users to discover and rate webpages, photos, videos, and news articles.These webpages are typically presented when the user who is known as a stumbler within the Stumbleupon community clicks the "Stumble!" button on the browser's toolbar. 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.