Any links that is crawled by google bot is called dofollow. Here is the example of DoFollow links:
Example of a normal link with DoFollow that will be followed
<a href=” target=”_blank”>http://www.example.com”>Link to example.com</a>
Google page rank value is depends on the counts of DoFollow links pointing towards any blogs. Let’s say, Blog A received 100 backlinks in 3 months with DoFollow, this will positively give effect towards page rank. Also, keep in mind that, any paid backlinks will face heavy penalty from google page rank.
Any links that is not crawled by google bot is called nofollow.
Example of a nofollowed link.
<a href=”target=”_blank”>http://www.example.com”rel=”nofollow”>Link to example.com</ a>
Google encourage all bloggers and sites to use nofollow for the links that you don’t want google to crawl/index. For this reason, by default wordperss and blogspot blogs comments are nofollow. However, there are some individuals still against this policy as they want to give credit towards their commentators. There are so many plugins available to make your blog comments section to dofollow. You can search it from google search or wordpress plugin directory.
Google introduced nofollow since blogging becomes more famous and comments are used to spam purpose to gain page rank juice faster. Google also announce illegal to sell or buy page rank links. So, if you have any paid dofollow links in blogroll and paid posts, please change it to nofollow as it may save you from the page rank penalty in next update.