The rel attribute is available for use in a few HTML tags namely the <link> and <a> anchor tags, but until recently it has been fairly pointless to use because web browsers did not support the intended functionality of most of the values you could assign to the rel attribute.
The rel attribute has been around since HTML 3 specifications and defines the relationship between the current document and the document specified in the href attribute of the same tag. If the href attribute is missing from the tag, the rel attribute is ignored.
<link rel="stylesheet" href="styles.css">
In this example the rel attribute specifies that the href attribute contains the stylesheet for the current document.
This is probably the only recognised and supported use of the rel attribute by modern web browsers and by far the most common use for it to date.
There are other semantic uses for the rel tag, beyond those which a browser might find useful; such examples include social networking, and understanding relationships between people; see http://gmpg.org/xfn/intro, the other use which has been talked a lot about recently concerns search engine spiders.
Search Engines and the rel Attribute
Recently Google has played a big part in finding another use for the rel attribute. This time the HTML tag in question was the humble anchor tag.
Google and the other major search engines (MSN and Yahoo!) have a constant battle with SERP SPAM which clutter their results and make them less useful. These pages make their way into the top results pages by using black hat SEO methods such as automated comment SPAM, link farms etc.
Rather than adopt a complex algorithm to determine these SPAM links which increase target pages search engine vote sometimes called "Page Rank" or "Web Rank", the search engines (Google, MSN and Yahoo!) have collectively decided that if blogging software, big directories and general links pages etc use anchor tags with a rel="nofollow" attribute those links will simply be ignored by search engine spiders, yet still be fully functional for end users.
Of course using rel="nofollow" does not mean the links are deemed as bad in any way, every link on a blog comment will be treated in the same fashion. The webmaster is essentially saying
"this link was not put here by me, so ignore it and do not pass any "link juice" on to it".
More on nofollow by Search Engine Watch.
Putting Webmasters in Control
Putting this kind of control in the webmasters hands hasn't been without controversy. People will always try to experiment with ways of manipulating the intended outcome to favour their own goals, such as using nofollow internally in their site etc. Others have welcomed the move as a way of reducing the problem of spamming.