Google assigns a "page rank" to every page on the web according to its "importance" in Google's eyes. This importance is determined by a few different factors.
1. The number of links to the page from other pages
2. The quality of the links to the page. Meaning, a link from a page with a high page rank is "worth more" than a link from a page with a lower page rank.
3. The relevancy of the site that is linking to the page.
3. The anchor text which is used for the links to your site
4. The keywords on your site, in the header and titles.
Google has a toolbar which you can download in order to see the page rank of every page you're on while surfing. It's hard to know how accurate the numbers on the toolbar are, though. They are only updated quarterly, whereas page rank changes daily.
If you want to read a great article about page rank see this great article called Page Rank Explained on Yaro Starak's blog.
Page Rank, Google, Yaro Starak, backlinks