Hlo Friends,
Differentiate between Dofollow and Nofollow?

 it's a good practice to use Nofollow link attribute to those link, where you don't want to pass link-juice. Dofollow links allow google (all search engines) to follow them and reach our website.

Do Follow - Do-follow links provide better "link juice" for improve your website in search engine rankings.

No Follow - Search engines only count do-follow links. If any link that is with no-follow attribute, its clear massage telling search engines to ignore it. No-follow links can often be highly context-relevant, and come from sources that are likely to generate greater levels of referral traffic.

Nofollow link attribute to those link, where you don't want to pass link-juice. Dofollow links allow google to follow them and reach our website

Very good Information. it is useful for me. thanks for sharing..

Dofollow Link is a hyperlink that is able to tell all search engines to pass along it's page rank influence to an outbound link. If you will use or get do follow links then search engines will count your link in the eyes of Google, yahoo, Bing etc as backlink for your site and you can get some positive value for your site ranking.
Nofollow is html tag that website owners, add to links in their website for the search engines not to crawl, notice, or acknowledge a link.

No Follow Link
A no follow link is a link that does not count as a point in the page's favor, does not boost PageRank, and doesn't help a page's placement in the SERPs. No follow links get no love. Theirs is a sad and lonely life.
A no follow link is created with the nofollow link HTML tag, which looks like this:

<a href="http://www.serped.com/" rel="nofollow">Link Text</a>
Do Follow Link
Before coming to the explanation of Do Follow link, here is how the normal link juice transfer takes place for dofollow links.
Dofollow link passes link juice and they definitely help you much, in order to achieve good page rank. As it instructs search engine spiders to follow this link.

<a href = http://www.example3.org  rel="external">Example3</a>

A DoFollow link passes the SEO strength or the Page Rank of the page to the website to which it is linked.

On the other hand, a NoFollow link is the link that is generally ignored by the search crawlers.

Do follow and no follow is part of backlink
Do follow is which link pass to search engine for backlinking
No follow is which link does not pass tp search engine for backlinking 
:p

The no follow links are not crawled by Google bots and those links does not pass any link juice to your site from where the back link is been located.
The do follow links will pass the link juice from the back link referring domain and it will tell Google bot to follow the link

No-follow links are the links that does not count as a point in the page's favor, does not boost PageRank and doesn't help to boost placement in the SERPs.
A no follow link is created with the nofollow link HTML tag, which looks like this:

<a href="http://www.website.com/" rel="nofollow">Link Text</a>

Dofollow link passes link juice and they definitely help you much, in order to achieve better page rank. As it instructs search engine spiders to follow this link.

<a href = http://www.example3.org rel="external">Link Text</a>

The only technical difference between the two is that a nofollow link has a nofollow tag. ... However, they ONLY count dofollow links in their algorithm. In fact, according to Google, nofollow links don't pass any PageRank. Google does not transfer PageRank or anchor text across these links.

TO get this answer you can check old posts on the forum because this question is asked number of times in this forum.

Getting do-follow backlinks helps you to get domain and page authority of that website to your website.