Nofollow is an html attribute first introduced in 2005 by Google. Originally, the rel="nofollow" attribute was added to discourage comment spam in blogs. Links with nofollow should not influence the link target's ranking in the search engine.

From the Google Blog:

From now on, when Google sees the attribute (rel="nofollow") on hyperlinks, those links won't get any credit when we rank websites in our search results. This isn't a negative vote for the site where the comment was posted; it's just a way to make sure that spammers get no benefit from abusing public areas like blog comments, trackbacks, and referrer lists.

From Google Answers:

"Nofollow" provides a way for webmasters to tell search engines "Don't follow links on this page" or "Don't follow this specific link."

So the rel="nofollow" attribute is a tool that can help define your most important links to the search engines.

Search Engines that support nofollow: Google, MSN Search and Yahoo!

History of Nofollow

Originally, the nofollow attribute was used on the page-level with a meta tag. It instructed search engines not to follow (crawl) outgoing links on a page. Example: see code example on main site

Another way to tell search engines not to follow (crawl) pages was to include them in a robots.txt file. But it was more difficult to prevent robots from following individual links on a link by link basis. The rel="nofollow" attribute allows webmasters to easily instruct the search engine robot to not crawl a link.

How to Spot Nofollow Links?

You can install browser plugins, or edits to Chrome to show a highlighted color over nofollow links. You can also install a plugin to your blogging software.

Nofollow Implications

Links are now either dofollow or nofollow. From the perspective of building links to your site or blog, dofollow links will always give you more link juice. While nofollow links won't either harm or help your link building. With that said, how you decide to build links with respect to the nofollow attribute (whether you want to get ONLY dofollow links or both to create a more natural array of links) will be an individual choice. How to use nofollow in link building strategy is a discussion for my next post!

Nofollow FAQs

How Do I Remove Nofollow?

1. Browser Plugins - highlights all links which are nofollow

- Chrome extension - add text to ChromEdit and follow directions

- FireFox has a version of the ChromEdit called ChromeEdit Plus

- Firefox plugin - SEO for Firefox

2. Blog Plugins

Wordpress Nofollow Plugins:

- Lucia's Linky Love - Rewards frequent commenters, you can set how many comments must have been made to be awarded a dofollow link.

- Nofollow Reciprocity - Detects nofollow links, if you link to a site and it is nofollow, your link to them also becomes nofollow.

- NoFollow Free - removes the nofollow attribute from blog comments and inserts an image band at the top of your pages with the phrase: “NoFollow Free” (optional).

- Dofollow - Simply install and activate, all nofollows are removed from comments and trackbacks.

- Dofollow (WP Plugin) - allows nofollows to be removed from comments, pingbacks and trackbacks but you can also set a comment age limit as well as have nofollow removed left by user type.

Andy Beard has a list of more nofollow plugins; including those for Blogger, TypePad and Moveable Type.

How Do I Add Nofollow?

For more control of your inbound and outbound links, you can add tag rel="nofollow" within any html tag. Example: see code example on main site

Does a link with nofollow count as a backlink?

Opinions vary on whether the nofollow attribute is completely discounted, or just given less relevance to the search engines. Keep in mind that how the search engines view nofollow may also vary. However, according to Google, that answer is no.

Why would I want to use nofollow on a link?

You might want to use nofollow on a link you don't endorse, but has information you want to point to. On blog sites, you may want to give a nofollow link to spam comments and give credit to comments that are well thought out or contribute to your post.

For more information, please see the entire Nofollow 101 post at the http://SearchEnginePeople.com blog.