del.icio.us is basically a way of sharing "bookmarks" online, allowing you to see the most popular sites related to various topics. You write tags for each of your pages (for example 'Cakes' might be tagged 'blog elearning technology higher_education'), so that people searching for those topics will see those pages.
If you go to the del.icio.us homepage and select one of the topics on the right hand side, such as web2.0. You can look through the sites most recently tagged as this, or the most tagged sites.
This is an alternative way of searching than Google, for example. It is interesting because, in the spirit of Web2.0, it is the user who is tagging the content, making up new tags (folksonomy) and recomending the content.