In one of my previous post, I have mentioned about microformats. I am not giving good attention to this topic; still I want to post here because it is going to be used in future. It is about web 2.0. So, you can learn slowly but surely about microformats.
Here you will find the possible and accepted values for 'rel' in links and link headers.
How is rel used / How is the 'rel' attribute used?
rel values indicate the relationship of the target resource to the resource on which they appear. In microformats, rel are used on links (e.g. <a rel="home" href="http://example.org">). Elsewhere, they are also used on link headers (e.g. <link rel="start" href="http://example.org/section/page1.htm">.
Multiple values may be used, in the same manner as multiple class values (e.g. <a rel="home me contents" href="http://example.org">).