You might have heard about increasing Google page rank with DoFollow Blogs. Before going into how such blogs and websites can help you in increasing your blog or website page rank, lets throw some light on the concept behind DoFollow and NoFollow.
NoFollow is the relationship attribute for hyperlinks in HTML (Hypertext Markup Language). HTML is responsible for rendering web pages in web browsers. These attributes tells search engine bots either to follow/crawl the associated Hyperlink or not. Above markup corresponds to a DoFollow link, which means the resulting link will be followed by Search Engine Spiders.
Now, If we put a NoFollow relationship attribute in this markup, the resulting link will not be followed by Search Engine Spiders:
with a particular Hyperlink, makes the link followed by Search Engine Bots, but such attributes doesn't exists in reality. Using such wrong attributes may cause a link to appear as SPAM to Search Engine Bots. If you want to make a link followed/crawled (DoFollow), simply don't use any type of relationship attribute with it.
Also Inbound or Internal Links are the Hyperlinks present in a particular page of your Blog/Website, which points to other Posts/Pages present within your Blog/Website, whereas Outbound or External Links are the Hyperlinks present in a particular page of your Blog/Website, which points to Posts/Pages of some other Blog/Website. Now, it is quite understandable that blogs and websites using no relation attributes in their links are DoFollows.
So does a DoFollow Blog increases Google PageRank of your website?
People, especially Webmasters, usually search for DoFollow blogs with high PageRank (PR) to submit comments on articles there. It's a basic link-building strategy. Also, Webmasters makes their blog DoFollow in order to increase responses and traffic!
If you're commenting honestly after reading an article, that's fine, but what if you're the one whose own blog is DoFollow? It is a firm belief among Bloggers and Webmasters all over the globe, that DoFollow links may PASS the Google Juice of a blog/page (Decrease PageRank).
Bloggers and webmasters have complete control over allowing or restricting Hyperlink(s) to be followed by Search Engine Bots, in there Posts and Pages. But when it comes to comments, it is the back end (template of the blog), which controls the link relationship behavior (which is nofollow by default). But, It can also be modified by means of some advanced editing or plugin usage.