From wikipedia: Social bookmarking is a method for Internet users to store, organize, search, and manage bookmarks of web pages on the Internet with the help of metadata.
In a social bookmarking system, users save links to web pages that they want to remember and/or share. These bookmarks are usually public, and can be saved privately, shared only with specified people or groups, shared only inside certain networks, or another combination of public and private domains. The allowed people can usually view these bookmarks chronologically, by category or tags, or via a search engine.
By participating in a social bookmarking network you have the ability to promote your own website, pages or posts along with other material that you might find interesting. Sharing your posts will drive more traffic to your site.
Different systems make suggestions to community members in different ways. For example Digg lists the latest or most popular links on its homepage. StumbleUpon will make suggestions to its users based on their preferences, other sites they have bookmarked or a site that the user is currently visiting. Delicious lets you click through to others who have bookmarked the same page; a great way to discover people with like-minded interests.
You can add widgets to your site which allow others to bookmark your pages or posts (such as add this or SocialList.org. You can also bookmark your content yourself, but make sure that you are adding other valuable bookmarks (of things that are not yours) in order to not be apparent in your self-promotion.
Social news sites allow users to post and vote on news stories, determining the popularity and visibility of news stories. It is the same tool as social bookmarking but is specific to news or informative stories.
If you have a story about your work or include informative or news-related posts on your site, this can be used to drive traffic to your site.
See “How” in Social Bookmarking