Here's how the del.icio.us site explains itself:
del.icio.us is a social bookmarks manager. It allows you to easily add sites you like to your personal collection of links, to categorize those sites with keywords, and to share your collection not only between your own browsers and machines, but also with others.The Beelerspace article goes into much more detail, however, and is much more effective at explaining why this is so cool.
Here's my del.icio.us page. (Related: see this Wikipedia article on "folksonomy," and this longer, more scholarly treatment of the subject by Adam Mathes.)