Sumbleupon is a type of social network whose main purpose is to allow users to share information and material rather than just chat. It was founded in 2002 with the general aim of improving people's online searching experience and allowing them to share what they found.
In some ways it works a bit like a library catalogue. The idea is that, instead of searching the whole web for content, you search in a place that helps you to narrow down your search so you get more relevant results. For instance, if you want images of mountains you could put 'mountain images' in a search engine, and you would get a lot of relevant content but also a lot of unrelated images. But if you put the same information in Stumbleupon you will get nothing but images of mountains - and if you specify, say 'mountain ranges' you will get just that. So it should save time and improve the quality and relevance of the content you get.
You can learn more about Stumbleupon here.