Social Bookmarking
Social bookmarking services is what happens when the internet is categorized by real people instead of Google's robots. People share their list of favorite and most useful web sites publicly on web sites like Del.ic.ous, Technorati, Stumbleupon, Digg, Spurl, Google Bookmarks, Yahoo My Web, or as you might have seen these various symbols on sites that you visit: 
.
When you click on of of those symbols and then take a moment to describe the site you are bookmarking, by organizing your own links, you are helping to categorize and organize the web.
The Democratic Republic of the Web
Link organized by real people are usually more relevant and are clustered around certain subjects. Sharing them helps others who surf find sites that are similar in nature. It's liking mining for gold and find a particular rich vein of sites.
Because social bookmarks work differently than than search engines in recommending the most relevant sites to visit, social bookmarks are a really good place to find sites that search engines miss and visa versa.
Why You Might Care
A social bookmark is vote for a Web site. A social bookmark helps others who use social bookmarks find important sites that the search engines miss. Same for you. Looking for a recommendation, search your social bookmarks. Looking for the best site on mountain climbing, or php programming, search the social bookmarks.
You might even find that social bookmarks return better results than most search engines. Or, you will most likely find that social bookmarks make for the a viable secondary place to search when the search engines themselves leave you cold.
A social bookmark travels well. You've got your private bookmarks on your computer, but synchronizing your private stash with your laptop, or second computer in your home, or taking them with you when you travel, is inconvenient.
Social bookmarks, on the other hand, go where ever you need them on the Internet. Portable, lightweight, and protected behind a username and password, you can get to them anytime you need them when you are on the Internet.
It's likely that the number of social bookmaking services will make you confused. So, look at a few of them, find the ones that give you the best result for what your interests..
Use one service to keep your bookmarks, then bookmark other services to use as a backup for search.
Here are some of the more popular social bookmarking sites. Try bookmarking this site, if you like it, but definately visit these sites and see which one works best for your interests.
For me, my two favorites are del.ic.ious and stumble upon. Stumble upon, despite its name retuns interesting, high quality sites 8 out of 10 times.