What is the role of Do-Follow links in page rank improvement?
Have you Googled this question? There are literally thousands of answers to this question online already. 
This question has been asked and answered many, many times before. Please read the SEO FAQ or use the [url=http://www.sitepoint.com/forums/search.php]Search facility to find past discussions.
If, after reading those, you’ve still got any specific questions that haven’t been answered, please do come back and start a new thread. In the meantime, I’m closing this one.