From their site: del.icio.us is a social bookmarking website -- the primary use of del.icio.us is to store your bookmarks online, which allows you to access the same bookmarks from any computer and add bookmarks from anywhere, too. On del.icio.us, you can use tags to organize and remember your bookmarks, which is a much more flexible system than folders.
The API provides read/write access to Delicious bookmarks and tags via an HTTP-based interface. The RESTful API returns responses in XML.
After our recent look at the growth of social APIs, we thought we'd look back at the history of the category. We added the Plazes API in September, 2005. In terms of the web, this is recent history. In terms of APIs, we might as well be looking at scrolls. In fact, less than half of the first 10 social APIs are still alive today. Below you'll find a list of those trailblazing 10, along with a status for each of them.