Do Follow: It allows search engine crawlers to follow the link for a good boost over SERP (Search Engine Result Page) and pass the link juice to the destination domain.
Dofollow is a descriptor of links describing that search engines crawl them and count them as votes of quality.
NoFollow: NoFollow does not allow search engine crawlers to follow the link and does not pass on the link juice.
<a rel="nofollow" href="http://xyz.com">abc</a>
* Link Juice:Link juice refers to the value or equity passed from one page or site to another. This value is passed through hyperlinks. Search engines see links as votes by other websites that your page is valuable and worth promoting.