First of all, what is social bookmarking? A quick answer is that it is an on-line way to save your Internet bookmarks. But it’s more than just having access to your personal bookmarks, it also means having access to everyone’s shared bookmarks and it usually entails being able to categorize your bookmarks and perhaps being able to view some type of popularity (or even rank posts as in the case of digg)
There are quite a few social bookmarking sites on the web including some you may already be aware of such as del.icio.us, digg and Furl. Social bookmarking is most often free and requires setting up an account on the site that you choose.
Del.icio.us is probably the easiest to get a grasp on. Say, for example, that you create a del.icio.us account and add a bookmark for Smalldog.com. You can then see how many others have bookmarked Smalldog.com (170 others) and then you can see what other sites these folks have bookmarked. Following links that like-minded people have will lead you to web sites that will probably interest you.
If you are completely new to social bookmarking, there is a beginners tutorial at Ma.gnolia
There is a review of 19 different social bookmarking sites which can be found here.