Social bookmarking is basically a way for people to share their favorite sites with their friends. So say you have an account with one of these SB sites and you have "friends" associated with your account. Every time you recommend a new site, your "friends" they are notified, etc. (There are so many sites now and they all work in different ways but that's the basics)
So as a Webmaster, you want to submit your site to these sites in hopes of others recommending you to their friends and getting exposure on the actual site. For example, when I write a blog post I always submit it to
www.digg.com. If other digg users find your article and love it they can click "digg it" and the more "digg its" you get, the more exposure that one article will get on digg.com.
It's really all a popularity contest. I've found that digg is best for articles with mass appeal (ipods, latest news, etc.) Stumbleupon is better for content oriented sites because the categories you choose for your site are much more specific so you have a better chance of attracting a more targeted audience.
I don't generate a ton of traffic from these sites but I get a few every now and then and I feel it's worth it. Once and a while I'll check my stats and all of a sudden I have 50 hits from Stumbleupon. Some people are heavy social bookmarkers and they use them all... digg, stumbleupon, furl, you name it. But I generally just use digg and stumbleupon.
Anyone else use SB sites?