PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page's value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at considerably more than the sheer volume of votes, or links a page receives; for example, it also analyzes the page that casts the vote. Votes cast by pages that are themselves "important" weigh more heavily and help to make other pages "important." Using these and other factors, Google provides its views on pages' relative importance.
So basically, every time I link to you, I contribute an iota to your page rank. I just contributed -- apparently -- to Tami's, Wikipedia's, and Google's.
I am curious though, how many links earn one a bump up? That is, I just went from 0 to 1. How many more links till I hit 2? Do you want to find out? If each of my readers links to one post, that would be lots of links. It would be fun to see how many links are needed to get 'promoted'.
Also, what use is a high page rank? So Google thinks my blog is important, or not. Why would your average blogger care? Does anyone know?
edited to add
I went exploring some more and found a post that describes how to modify your template so that everyone that comments on your blog gets a rankable link. (?) I think I implemented it; you'll have to let me know. I'm still wondering what the purpose of this is.