nofollow link means that when search engines bots analyze the site content; it says that to search engines bot "Don't follow this link."
This is useful for keeping the search engine on your page and for maximizing the "link juice" on your page.
nofollow tag indicates that the appearance of the link is not a vote for the relevance of target site to search engines.
nofollow links mostly used in social media sites, prohibiting posters to boost their website rankings by simply pasting links to their websites.
As you know google calculates site's ranking with using the count and quality of links from other site. So when search engines find the nofollow tag on link HTML code, they don't use this links on rank calculations(especially google, other search engines use nofollow links in a different way.)
There are 2 ways of using nofollow code on a page.
1 - nofollow attribute can be appeared in the page meta tag, and tells that to search engines not to follow(crawl) any outgoing links on the page. Usage sample:
<meta name="robots" content="nofollow" />
2 - Or you can use just for specific links. It tells that to search engines bot, "do now follow this link and do not use it on rank calculation". Usage sample:
<a href="login.aspx" rel="nofollow">login</a>
<a href="www.unimportantsite.com" rel="nofollow">XYZ</a>
How does Google handle nofollowed links?
In general, google don't follow them means that Google does not transfer PageRank or anchor text across these links.
Why do we need to use nofollow links: Following there are several examples need of nofollow links.
- Untrusted content: If you're not sure that the content of the site. For example bloggers can not be sure the quality of content of the page for blog comments. Using nofollow links on blog comments helps you to block comment spammers.
- Paid links: It's a good way to use nofollow links to prevent paid links from influencing search results and negatively impacting users.
- Crawl prioritization: Search engine robots can not login or register as a member on your forum, so there's no reason to invite bots to these kind of links. Using nofollow on these links enables bots to crawl other pages you'd prefer to see in search engine's index.