Sculpting PageRank? Are you scratching your head, wondering whether I'm talking about carving a masterpiece? In a way, I am. I'll get to why that is, in a minute.
PageRank, a Google-trademarked term, is a measure of the importance of a page. It's actually a lot more than that—it is the foundation of Google's ranking algorithm (stated as such by Google on its Google Technology page). Because of the key role that PageRank plays in search engine optimization (SEO), it makes sense to make the best use of the PageRank that your site possesses.
A site's home page typically accumulates the most PageRank (or "link juice," if you want to speak in terms that include other major engines). That's because most inbound links point to a site's homepage rather than deep into the site.
How you "spend" that PageRank throughout your site—in other words, how you flow your PageRank deeper in your site—is determined primarily by your internal hierarchical linking structure. That's why having site-wide navigation that's via keyword-rich text links rather than graphical buttons is so critically important.
So where does "carving a masterpiece" come in to play? It's based on the idea that you have a way to shape how PageRank is distributed across your Web pages via more than simply changing your navigation hierarchy. It relies on a tool, provided by the search engines, called "rel=nofollow." With it, you can send your PageRank flowing through some links, but not others.
Think of your site's navigational hierarchy as your blunt instrument, and the "rel=nofollow" attribute as your scalpel.
This level of control is a relatively recent development. The "rel=nofollow" attribute was first introduced in 2005 as a way to discourage comment spammers from defacing blogs with their links and bogus comments; the use of nofollow connoted that you did not vouch for the quality of the page being linked to.
But the engines have since evolved their thinking. They realize now that rel=nofollow is a much more versatile tool than when it was first conceived. Consequently, the use of nofollow has been expanded so that it doesn't connote anything negative whatsoever, as confirmed by search engine representatives on multiple occasions (such as in this interview with Google's Matt Cutts). So now you can feel free to use "nofollows" within your own site.