In Layman language DoFollow link are those links that passes page rank and NoFollow links doesn't passes its page rank. i.e. you can also say dofollow Links: will pass on the SEO benefits of the website from where it is built from and NoFollow doesn't.
Google has announced in 2005, about this no-follow, Hyperlinks with rel=”nofollow” would not influence the link target Page Rank. A do-follow link will pass the SEO strength/Page Rank of the page to the site that it links to. A no-follow link, in theory, will not do this.
It prevent robot from following individual link that exist in a page. Look it's like you have a page and I came down to your page and you have implemented dofollow in the comment box, I comment on you page and drop my link there now all the SEO benefits will pass to my page i.e. either its good or bad but if you have put Nofollow tag and I drop my link on the comment box then, robots will not consider that link "which I have dropped on your website" as your's and will not pass any SEO benefits to my link. It is usually use to stop spamming what many other people previously do is they i.e before the introduction of NoFollow tag, suppose your site have high PR, good traffic etc... what they were doing is they drop their link on your site and as the robots consider that link as your site it passes the Page Rank and all the benefits to the other site. Another demerits of it is if you are involve in some black hat technique and you site got pennalzes then my site is also going to get penalize. So, the merit is even if I am not working hard I can get to top just by having a link on you site and no matter how much quality work you have done, you will get penalize if i am involve in some ...............
Do follow link is more effective and it passes page rang and no follow link does not pass page rank. You should prefer to do any submission in do follow website.
Dofollow links is that search crawler follows and nofollow links not crawled by search engine crawler.
If I put nofollow links in particular keyword then it will boost that keyword up on the Google otherwise it doesn't effect on that particular keyword .
Well, I don't think that it is going to show any effect !! NoFollow and DoFollow tags are for links i.e. are they going to get any benefits or not!! I don't seem to get putting NoFollow link in particular keyword !! Well what I am getting by looking at your answer is you need to get cleared a bit more try googling round, that might help you
Basically search crawler not crawled the link with nofollow tag, means that Google does not transfer any PageRank or anchor text across these links but it can be useful for the site in visitor point of view.
In Do follow search engine crawler crawled and do indexing but in No follow search engine crawler crawled but dont do indexing.
Do follow links are increase your page rank. No follow and Do follow links are both important for crawling and indexing of the website to the search engines.
Nofollow tells google that you're not vouching for the next to that website. Google WILL crawl the nofollow link but will not pass page rank to it.
Dofollow links helps your improve page rank and also ranking in search engines and Nofollow link crawled but not help in page rank and ranking it's help drive traffic to your website..
Do follow link pass the reputation while no follow doesn't pass the reputation. To have a page rank one should balance both the links in his websites. Both the links are important for page rank.
A dofollow link will proceed the SEO quality of the page to the website that it links to. A nofollow link, in theory, will not do that . That's the difference between the two.