nofollow:-
These links does not provides link juice.
dofollow:-
These links provides link juice and very helpful in increasing the traffic of a website.
Yes, You can also do it using a Simple Add-ons in Firefox Browser. Just Search for "NoDoFollow 1.1" and Install it on your Firefox Browser. After that, Restart your browser and right click on your mouse in browser and click on NoDoFollow. You will see Nofollow link in Red Color and Dofollow link in Grey Color.
Do-follow links are very important in SEO. Only do-follow links are crawled by google. which helps to increase page rank and traffic.
If your backlinks are followed, search engines won't crawl your backlink on the page.
A do-follow link improves your website rank and domain authority with the link juice.
A no-follow link on the other hand, does not benefit you with the link juice.
That is to say, while a no-follow link does not provide any benefit in SEO, a do-follow link does.
You can identify a no-follow and do-follow link with the page it directs you to or through the page source html codde.