Del.icio.us is a social bookmarking tool that allows users to organize and share bookmarks across platforms and search bookmarks tagged by keywords. It offers advantages like being ubiquitous, organized, searchable and integrated. Users can see bookmarks liked by others in their network and view tag ratings from other users, and can have research done for them by subscribing to tags. The document provides instructions on setting up an account and using the basic features of adding sites, tags and URLs to bookmarks.