In the social web, this is a way to represent a hyperlink, making it more valuable and informing machines/bots about the link Read this for more details http://en.wikipedia.org/wiki/FOAF_(software) or http://en.wikipedia.org/wiki/XHTML_Friends_Network
Check this thread: http://forums.digitalpoint.com/showthread.php?t=1752768 This is the XFN link system. XFNâ„¢ (XHTML Friends Network) is a simple way to represent human relationships using hyperlinks. In the thread above I explained it and put some references to get more detailed information.
This make link more meaningful for search engines. There are other types of RDF That are many similar tags
The rel attribute specifies the relationship between the current document and the linked document - http://www.w3schools.com/TAGS/att_a_rel.asp