In simple terms, DoFollow is the default state for a link. It means any link without any no-follow attribute is the dofollow link. Do Follow links are introduced in 2005 by Google, are an HTML attribute that reduces the search engine spam, allow search bots to follow the links and improve the search engine results. Do Follow link is the only link that can pass link juice to the linked webpage and influence the targets page rank effectively.
On the other hand, No Follow Links have a very little value that does not count as a point in the page’s favor, doesn’t boost page rank nor help page’s placement in the SERPs. A nofollow links include a small piece of code, called an attribute that looks like this
This tag lets the search engine know not to follow the link. This link lowers the effectiveness of search engine spam, averts spamdexing from occurring in the first place and, enhances the quality of search engine results.