In short Page Rank is a "vote", by all the other pages on the Web, about how important a page is. A link to a page counts as a vote of support. If there's no link there's no support (but it's an abstention from voting rather than a vote against the page).
True, but I think there is no definite answers. The only definite answers will be how google see's your site.
Page rank is given to your site depends on how much quality back links and traffic you have to your site. To have high PR you should have quality back links.