what is the difference between in dofollow and nofollow. i have some nofollow backlinks will these help me in ranking
Dofollow and nofollow - both type of links actually help, but in different way. But Do follow links are much better than no follow links although they hold some importance. A dofollow link means that search engines will follow that link and give you link juice. But a no follow link will not give you that advantage. But definitely a link on any other page whether it is do follow or no follow matter a lot. Because it increases your link popularity.
Nofollow links are good for yahoo and bing. Nofollow links are good also for getting popularity for your site and trust ranking if they are placed in high authority websites, like twitter, facebook, stumbleupon, etc
Nofollow gives no linkjuice attributed to the anchor link. Dofollow is the default setting, which does give linkjuice. From personal experience, I think Google is currently giving some credit to nofollow links. They cycle in between nofollow weights, so I wouldn't worry too much about it.
no follow not suitable tag for search engine, search engine understand the easily do follow sites, and give high page rank.
Nofollow are for not crawling a link by search engine. Dofollow allow search engine to crawl links. Nofollow tag we can use if we are using a link for reference not for crawl. Both are good for human traffic