Page Rank. Google assigns each webpage a number from 0-10. This number basically shows you the popularity of that page based off mostly of incoming links to that page. The more incoming links the more page rank you get. As you get higher up in numbers it is harder to get a higher page rank. You can see your page rank on the google toolbar. Download and install it. Page rank is only updated approx. every 3 months in the google toolbar. *note Pagerank does not mean anything. Dont get obsessed with it.
But i think above information is incorrect. look at the PR of http://www.bit.lk/ that is just the website of colombo university. they don't have any back link. but PR is 5