[marknel]

"A tag is a keyword. It is created by adding the attribute rel="tag" in any link going any where.

<a href="http://lorelle.wordpress.com/" title="Lorelle on WordPress" rel="tag">Lorelle on WordPress</a>

This creates a “tag” for “Lorelle on WordPress”. The tag is the words which the link wraps around. Without it, it’s just a link. No more, no less.

With it, it is recognized by search engines and tag services crawlers which add that word or phrase to their “tag database”, acting like a keyword. When a visitor types in the tag word or phrase in a search engine or tag service, like Technorati, any post with that “tag” or those keywords, even if not tags, will turn up in the results."

http://lorelle.wordpress.com/2006/07...-in-wordpress/