Social bookmarks are web page bookmarks that are saved online. Basically, they have the same purpose as the bookmarks (or favourites) in your browser, with one important difference: They are always available to you, doesn’t matter which browser or computer you are using.
Why should I use social bookmarks?
With the help of social bookmarking services (such as Digg, Misterwong, Del.icio.us, and many more) you can bookmark interesting pages or websites that you visit often and access them from every computer.
But that’s not the only advantage of social bookmarking: They also let you share your favourite sites. You can add tags (keyword descriptions) and comments to your bookmarks, allowing other visitors to find your favourite sites easily. And, of course, you can use them for your own website too. With accurate tags and comments, other users will find the way to your site and this will improve your ranking on search engines. Read more about it on Tips&Tricks
Here’s a great video explaining all you ever wanted to know about social bookmarking in less than three and a half minutes: