The name PageRank is a trademark of Google and is assigned to Stanford University.

Google's description of PageRank:

"PageRank relies on the uniquely democratic nature of the web by using it's 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 more than the sheer volume of votes, or links a page receives; 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'."

so.. If there are no links to a web page there is no support for that page AND there is no PageRank!

Primarily Google uses the number of links there are to the page in question to determine PageRank . As you acquire links - especially from other sites related to yours, and those with good PageRank - your PageRank will increase.

As the name implies, PageRank ranks pages and not websites. Your home page will probably have the highest PageRank of all the pages on your site. It is possible to have higher page ranks on pages inside a domain root page.

I consider PR 0-3 to be low, 4-5 to be good/average (and attainable), 6-7 to be above average and 8-10 to be high.