Linking is the piece of HTML code added to an anchor text that tells the search engines whether or not it should pass authority along the site or not.For that purpose In 2005, Matt Cutts, the Google’s webspam team head and Jason Shellon of Blogger brought up a new attribute in HTML codes i.e NOFOLLOW.
"Nofollow is a HTML tag added to the link anchor text" Like
<a href=”your link URL” rel=”nofollow”>your link name</a>
This is automatically done. means when you comment with the link, the NoFollow attribute will be added automatically.This link will produce less value and search engine crawlers would not follow them to index. So, such a link is useless as a piece of text.
The Do-follow backlinks get followed by the Search Engines and authority is filtered down to your site. It is the default attribute on any link, so it has the same effect of a normal backlink.It will have the attribute “rel=’dofollow’” instead of nofollow It is written as:
<a href “Your link URL” rel=”dofollow">link name</a>
Following video will help you to distinguish between do follow and no follow links, and also you will get complete knowledge of FireBug (Firefox add-on).