Hi I would like to know that what is the difference between nofollow comments and dofollow comments. How do they effect the link building and which option should be the best to get a backlink for the website. thanks
Definitely go for "dofollow" if you can. The only problem, though, is that I'd say about 5% of the blogs you'll come across are actually dofollow. Really, the best way to find DF blogs is with the help of software. (Scrapebox.) It has helped me build my own database of over 50,000 dofollow blogs. And that's not including blogs related to niche's I'm in. "DoFollow" passes the PR / link juice through your link. "NoFollow" does not.
When you comment on either a no-follow or do-follow blog and your comment is of quality, the chance of getting a backlink from that blog is high (do-follow ) or getting a quality consideration from Google despite of being a no follow link at all.
If you want page rank effects from your links, you need them to be on dofollow blogs. If you want to build traffic, however, both dofollow and nofollow links are equally good. In fact, if you want to generate traffic through a specific link, you're better off with a well-known and high-traffic nofollow blog than a dofollow blog that nobody reads.
The combination of DoFollow and NoFollow backlinks are good for ranking good. Because it looks natural to the search engine. On the other hand both backlinks provide valuable traffic. So both are required for websites.