People who belong to different social bookmarking sites....like Digg for example, click on the link and add it as a bookmark in that site. They do so not only so they can find it again, but also to share it with others in that bookmarking site. It's basically a social networking tool for things you are interested in and things you don't want to forget about. Many of these sites have a lot of additional features as well. Here a wikipedia link if you want to learn a little more:
http://en.wikipedia.org/wiki/Social_bookmarking