What is nofollow links and dofollow links

Basically if you created a link than by default it is dofollow link but if you add rel="nofollow” attribiute than it becomes nofollow link.

Example of Nofollow Link:
<a href="http://www.google.com/" rel=”nofollow”>Google</a>
Example of Dofollow Link:
<a href="http://www.google.com/">Google</a>

Whenever google bot crawls a page and it reaches a link, than as the name suggest if it is dofollow link than bot crawls the links and lands to the linked paged but if the link is nofollow than bot don’t crawls the link and doesn’t follow the link.

How does it effects your website/blog page ranking SEO

Backlinks are of great importance in optimizing a website or blog for search engine. The most important factor behind your page rank are backlinks. To get good page rank and rank high on google then you have to focus on building backlinks preferably dofollow backlinks as because whenever any of your dofollow link is crawled from a higher ranked page than your website/blog gets benefit of its page rank and its reputation is increased in google index.
But if your website or blog backlink on high page rank is of nofollow attribute than it will not count in making google high page ranking.