What page rank refers to is your ranking in a Google Search. This is why people Spam blogs. To get their own page ranked higher.
This is a bit of an oversimplification but this is how it works
Google searches are based in terms of the number of other websites that link to yours. Like if you have a blog and somene puts in a comment and links to his own website.
The original and current intent of Google is a link is an endorsement of that site by you. In otherwords Google says if you as a webmaster takes the time to put a link to another site that site must have relevant information to your readers. In other words you endorse that other site.
You can see how with a blog that intent gets lost.
Not all links are the same. For instance if you link to my page it is MUCH different than if the New York Times puts a link from its site to my site. Why? Because for most words the New York Times ranks #1 in a Google search.
So Google give the New York Times a high page rank. By linking to my site Google give a bit of that high page rank to my lowly site. So when Google reindexes my site gets a boost and will be ranked higher in a Google Search.
Because it shared that high page rank with me the New York Times rank goes down a bit because it gave part of it’s page rank when it linked to me. For the New York Times it means nothing because it’s HUGE.
But if ranking high in Google is a concern you don’t want to give away your rank just to anyone. Cause for smaller sites one link can mean the differene between appearing on page 1 or page 3 of a search. Let’s face it when we search how many of us look on page 3 of a Google search.
Now you can see why spammers put anything in blogs. To raise their page rank in Google.
BUT if you can make it so any links in your blog contain an HTML code that says <no follow>. What does that mean. To your readers it means NOTHING. They will still see the link and be able to click and go to the site. But the Google robot or spider (Called Googlebot) will NOT follow that link when it reads your site and WILL NOT give page rank to that site.
So by using the <no follow> attribute you are allowing your readers to go to the site but saying to Google I do NOT endorse this site.
MSN and Yahoo work similarly.
So if people know in advance there is a <no follow> they won’t waste their time putting garbage in your blog just to increase their page rank
Like I said I oversimplified a bit but that is the jist of how it works.
Go to http://searchenginewatch.com to learn more.
(And you will note that since the Straight Dope Message Board does NOT allow Googlebot to even READ their site) that link won’t be indexed