XHTML Friends Network (commonly abbreviated XFN) is a basic but useful way to represent human friendships and other relationships using common HTML hyperlinks. It is more of a technique than a new technology.
The technique was initially designed to deal with the rapid growth of Blogs and Blogrolls which, at the time of its creation, were the fastest growing areas of the Web. XFN enables web authors to indicate their relationship(s) to the people in their blogrolls simply by adding a 'rel' attribute to their <a href> tags.
Though the technique is a quick and dirty way to create links between friends across sites, it is quite possible that more recent social network mapping developments such as FOAF, OpenID and OpenSocial may force XFN into obsolescence. As such, its use should be strongly considered for alternatives/implications.