dofollow are counted by google nofollow are not counted by google It's all about SEO http://en.wikipedia.org/wiki/Nofollow
dofollow links are just your normal link eg <a hre='...'>...</a> and will be counted by google - help you with seo where as no follow links are not counted by google. An example of nofollow is <a rel=nofollow href=........ If you want to see the dofollow or nofollow links I suggest you have a look at NoDoFollow firefox addon.
If you want to drive traffic to your website through Google, then you need to watch out for do-follow and no-follow links. No-follow links are not counted by search engines and do-follow links are counted (for backlinks). If you want to get back links, and use Google to get traffic, then you would only comment on do-follow blogs. You can check for no-follow and do-follow links by the NoDoFollow firefox addon also mentioned by Skuvnar. I you do not care about getting traffic through search engines, then you do not need to consider the no-follow and do-follow links. For example, I do not use Google to drive traffic to my squeeze page. I use blog commenting and Forum Marketing. I am doing blog commenting on a number of blogs that are no-follow, and I get decent amount of traffic from it. Besides, you are building a relationship with the blog owner. Also, I just enjoy doing blog commenting, and so I do not get into the do-follow vs no-follow debate.
good explain by nab! in addition, if u put do follow on your blogs, then u're encourage people to put comments on your blog...but need to use spam filters...