The a and link elements can take rel and rev attributes to define the meaning of the link relationship. In the case of link, some special meanings (such as rel="stylesheet") are understood by the browser, but in general most values don't have any meaning to the browser (on exception is the Macintosh iCab browser, available at http://www.icab.de).
Early on in the development of HTML, Murray Maloney and Liam Quin wrote up a draft specification for rel/rev values, which has served as the foundatation for many current implementations. This draft specification never became a standard, but you can still access it and get an understanding of the different value and what they were intended to mean. This document is available at:
- http://www.oasis-open.org/cover/maloneyQuinLinkRel.txt, or
- http://www.ics.uci.edu/~ejw/authoring/draft-ietf-html-relrev-00.txt
A few years ago, I wrote up a longer document explaining some of this material in more detail, and providing some examples of recommended usage. This is also not a formal standard, but might be of interest if you want to know more about using rel/rev values in your own Web applications. This document is available in several Word processor formats: