PageRank is calculated per page, not per domain, so it’s quite possible to have different pages on the same site with different PageRank.
PageRank is based largely on the number and quality of incoming links to your page, and you’ll find plenty of threads here discussing it, if you care to look. However, Google themselves have said that they are giving less and less weigh to PageRank in ranking a site, as it’s too easy to try to manipulate it, so I’d advise you to put your energies into something more productive than chasing PR.