There's a lot of misinformation here, clueless people answering other clueless people. Technically there is no such thing as a do-follow link, there are simply ordinary links and no follow links, which are links with a rel="nofollow" tag added to the code. Search engines will crawl links that have no follow tags to see where they go, but will (allegedly because we'll never know for sure) not pass any search engine ranking power through them. Also, saying that all ordinary (or what people call do follow) links will pass on search engine ranking is frankly ludicrous, and if you still believe that you've probably never heard of Google's Panda or Penguin. Links from crappy websites, link farms and identical comments in the thousands will not add any 'link juice' to your site, and will in fact drain your site - often to the point where it can't be found on the first 200 pages. Google is trying to push the idea of website owners concentrating on their own website content and not thinking about links at all. If they suspect you're involved in creating link campaign to your own site specifically to boost your own rankings they will dump you into obscurity. That's why thousands of people are now frantically trying to get ordinary (do follow) links to their sites from low quality sources removed.
The difference between a Normal Link and a DoFollow Link is that the DoFollow Link has a special HTML Markup inside that tells the Search Engines like Google to FOLLOW the destination inside the link. Now have in mind that using this special markup or not will make the link a DoFollow Link automatically.
There are links without rel="nofollow" attribute. There's no reliable opinion do they make any influence on ranking or not