Hi, i will post a short explanation, that i find from Matt Cutts, specialist in google.
So should you use the nofollow attribute at all in your internal links? Google’s webmaster site recommends the use of nofollow in some scenarios:Search engine robots can’t sign in or register as a member on your forum, so there’s no reason to invite Googlebot to follow “register here” or “sign in” links. Using nofollow on these links enables Googlebot to crawl other pages you’d prefer to see in Google’s index.
This stand is however completely different from what Matt Cutts has suggested in one of his recent videos.
Matt has unequivocally advised that web publishers should not use rel=nofollowwhen linking to any of their internal pages even if they don’t want to see those pages in search engines.
He didn’t share any specific reason that made Google to change their original stand on the use of nofollow in web pages but, as a publisher, what matters more to us is the current recommendation which is ” no nofollow on internal links.”