What is the difference between Do Follow & No Follow Sites.

"Nofollow" provides a way for webmasters to tell search engines "Don't follow links on this page" or "Don't follow this specific link."

 

There are two kinds of backllinks do-follow links and no-follow links. Difference is that no-follow links won't contribute towards your PR. So it's obvious do-follow links are the required ones; but making only do-follow links can put you under google's suspicion so you must also make no-follow links but in lesser ratio.