A nofollow attribute was approved for use with anchor tags (hyperlinks) in 2005. As soon as it was approved, all the major search engines announced that they recognize and honor it. This is how it works.
The nofollow attribute is an attribute of the anchor tag and not a tag in itself. Although incorrect, it is frequently referred to as the nofollow tag. The anchor tag is the HTML tag that is used to form a hyperlink. An attribute modifies the tag.
To use the nofollow attribute, simply insert the following attribute code into any hyperlink code.
The name “nofollow” is a bit of a misnomer, because we often see search engine spiders follow these links. However, they are supposed to disregard the link, which means that it will not count as a backlink for the web page that the link points to. It also prevents a site from inheriting any penalty that may be applied to a disreputable or questionable site that is linked to.
Google has officially confirmed that links containing the nofollow attribute do not pass PageRank (commonly called “link juice”) or anchor text. You can find a lot of details regarding the nofollow attribute on Google’s page called About nofollow
PageRank or link juice is a value passed from the linking page to the receiving page. The higher the PageRank of the page containing the link, the greater the PageRank value that is passed to the receiving page. When no PageRank is passed, the link is rendered theoretically useless as a method for improving rank positions.
Anchor text is a very important aspect of link building, because the text used in a hyperlink reinforces the keyword theme for the receiving page. This helps the receiving page to rank higher for that particular text when used in a search.
When You Should Use the nofollow Attribute
As Google points out, the nofollow attribute should be used when linking to untrusted content, when using paid links, and when blocking spiders from pages in a site that you do not want them to crawl.
Untrusted content on another web site can be a big issue, because a site inherits part of a penalty that may be applied to a site that you link to. This includes porn sites, malware sites or any sites that use black hat methods that result in penalties. Google refers to those sites as “bad neighborhoods” in their Webmaster Guidelines.
Google has also been on a vendetta to identify and penalize web sites that sell paid links that pass PageRank. While all search engines encourage web site owners to pick up backlinks (inbound links) to their sites, Google has decided that they do not approve of sites that sell such links. They want to see natural links to a web site, not artificial links intended to manipulate PageRank. While you may or may not agree with this, they do actively penalize sites that make it obvious that they are selling links that pass PageRank. If you are going to sell links, the best solution is to sell them the basis of traffic and not PageRank that might be passed. The solution is to use the nofollow attribute in all paid links to other sites.
The third reason is to prevent spiders from indexing pages is your site that you do not want them to index, such as a login page.
Here is a video with Matt Cutts, one of Google’s Quality Engineers, discussing the use of the nofollow attribute.
What is the dofollow Attribute?
There is no such thing as a dofollow attribute. In web user jargon, the absense of a nofollow attribute makes a hyperlink a dofollow link, which means it does pass PageRank and anchor text. When you see someone refer to a blog as a dofollow blog, that means that any comments of links you add to any comments that you add to the blog are links that will pass anchor text and PageRank. Many blogs, including WordPress, add nofollow attributes to user comments by default and this is disabled on a dofollow blog.