The NoFollow tag (rel="nofollow") is an HTML tag that tells search engines to bypass the link during indexing.
In other words, search engines such as Google, Yahoo! and MSN crawl the web periodically to locate and index new pages to include in keyword search results. Each link on a web page is followed during the indexing process. Pages with many incoming links (particularly from authoritative websites) are ranked higher in searches than pages with few incoming links. If there are links on your blog that you don't want search engines to find, use the NoFollow tag to make them invisible in terms search indexing and ranking.
The NoFollow tag has become popular over the course of the past few years as spammers have tried to boost the number of incoming links to their sites by participating in comment spam, paid text link advertising, paid links in sponsored reviews, and so on.
Search engines don't want to give sites that artificially inflate their number of incoming links a higher ranking. Therefore, sites that participate in these types of link building activities are typically penalized by search engines. For example, Google removes advertisers, publishers and spammers who try to artificially boost incoming links by dropping the page rank for participating sites and possibly removing those sites from search results entirely.
Some blogging applications are set up by default to use the NoFollow tag in comments. It's up to you to decide if you want to use the NoFollow tag in comments on your blog or not. It's also up to you to decide if you want to participate in monetization activities such as paid link advertising and sponsored reviews without using the NoFollow tag.
