dofollow = a link where the search engine spiders will follow, count this backlink and link juice will be passed. nofollow = a link where the search engine spiders will not follow, the backlink will be shown in webmaster tools but no link juice will be passed.
Wrong, nofollow is good for seo... if every single one of your backlinks is dofollow then google will see this. A backlinking profile with only dofollow links looks unnatural and will likely get your site reviewed and potentially penalised.
Do follow links are those which are crawled by search engine crawlers. No follow links are not given much importance by search engines. However this days it is really hard to get do follow backlink so it is better to aim for quality related backlink.
When you link your site to a do-follow site the html link code will be a regular anchor text html link. In the case of a no-follow the no follow html tag will be added within the link and you would not get a link back to your site from this site.
http://forums.digitalpoint.com/showthread.php?t=2419910 i think this thread will help you to get clear idea....
The nofollow links are those ones through which the crawler can't go but that does not mean that there is no use of no follow links! the reason I am saying is that Google crawler identifies the no follow links hence to make the natural link building it is necessary to have both no follow as well as do follow links!! Thanks
nofollow : nofollow links didn't crawled by Google and not count as backlinks. dofollow : dofollow link crawled by Google as well as count as backlinks.