<!-- Imagine the current page is a contact page -->
<!-- The rel attribute -->
<!--
Describes the relationship between the current document and the page linked to in the href.
e.g. From the current page(contact), the page being linked to is it's help page.
-->
<a href="http://mydomain.com/help" rel="help">Help</a>

<!-- The rev attribute -->
<!--
The opposite of the rel attribute. The relationship between the linked document(help) 
and the current document(contact).
NOTE: This is considered overkill. rev is mainly used in the vote-links microformat.
-->
<a href="http://mydomain.com/help" rel="help" rev="contact">Help</a>

Rel and rev attributes are quite hard to get your head round, and very easy to mix up. Hopefully this snippet will explain the difference.

Difference between rel and rev attribute

HTML