Delicious is a social bookmarking site, which ultimately helps promotes your website. The First part is on a subdomain, the icio is the domain, and the .us is the domain extension (like .com, .net).

How Delicious works:

After registering for an account, when you see a website you like, you can bookmark it online at Delicious. As you bookmark it you give it a title and tags. The tags are ways to categorize your bookmarks. You can also join other people’s networks, which allows you to share your bookmarks with others. Delicious also also shows bookmarked pages publicly in their directory and search engine, so you can find other web pages by popularity of how often they were bookmarked.

Delicious plugins:

Delicious is a popular tool for webmasters to help facilitate promotion. You can get a delicious bookmark button for your wordpress blog, by getting one of the social wordpress plugins, which will include a Delicious button among others.