Social Bookmarking websites are basically sites that categorize and store 'bookmark links' (links which take one to a specific website). Millions of visitors flood these Social Bookmarking sites to search for information.
So the idea of bookmarking is to post links back to your web page from these Social Bookmarking websites. You need to specify some keywords (we call them TAGS) which categorize your bookmark.
This allows others to search and even vote for your bookmark. Social Bookmarking is in a league of its own because of its ability to generate traffic by sharing links.