The difference is that a nofollow link doesn't pass any pagerank to the page the link is pointing to. A dofollow DOES pass pagerank. You code it as follows: NOFOLLOW: <a href="..." rel="nofollow">Anchor text</a> DOFOLLOW: <a href="...">Anchor text</a>
do follow ...you get pr juice + back link nofollow....you just get link there would have not been pass any prjuice to your site.
the do follow sites keep your link and provide back link for good PR .. but no follow links behaves like a scrap keep in trace box .. it is useless..