From their site:
del.icio.us is a social bookmarking website, which means it is designed to allow you to store and share bookmarks on the web, instead of inside your browser. This has several advantages.
First, you can get to your bookmarks from anywhere, no matter whether you’re at home, at work, in a library, or on a friend’s computer.
Second, you can share your bookmarks publicly, so your friends, coworkers, and other people can view them for reference, amusement, collaboration, or anything else. (Note that you can also mark bookmarks on del.icio.us as private — only viewable by you — if you like.)
Third, you can find other people on del.icio.us who have interesting bookmarks and add their links to your own collection. Everyone on del.icio.us chooses to save their bookmarks for a reason. You have access to the links that everyone wants to remember. You can see whether two people have chosen to remember a link, or whether it was useful enough for a thousand people to remember — which may help you find things that are useful for you, too.