People think about PageRank in lots of different ways. People have compared PageRank to a “random surfer” model in which PageRank is the probability that a random surfer clicking on links lands on a page. Other people think of the web as an link matrix in which the value at position (i,j) indicates the presence of links from page i to page j. In that case, PageRank corresponds to the principal eigenvector of that normalized link matrix.SEOmoz | Google Says: Yes, You Can Still Sculpt PageRank. No You Can't Do It With Nofollow
My boss showed me about this by using a client example a few months back. After dealing with the page sculpting first hand, this post made a lot more sense.Robots.txt vs Rel=Nofollow vs Meta Robots Nofollow : SEO Book.com
Aug 6, 2008 - Aaron Wall's chart comparing elements
SEO hard to understand
I was just fixing up our Robots.txt tutorial today, and figured that I should blog this as well. From Eric Enge's interview of Matt Cutts I created the following chart. Please note that Matt did not say they are more likely to ban you for using rel=nofollow, but they have on multiple occasions stated that they treat issues differently if they think it was an accident done by an ignorant person or a malicious attempt to spam their search engine by a known SEO (in language that is more rosy than what I just wrote).