PR is a calculation algorithm of page authoritativeness used by SE Google. PR has own scale from 1 to 10, PR characterizes "importance" of page. "Weight" of the page A is defined by weight of the transferred page B link. It means that PR is a method of page weight calculation in a way of importance calculation of its links.
Google considers not all links. Links are filtered from sites specially intending for their congestion. Some links can not only not be considered, but also affect linking site ranging negatively (such effect is called as search pessimization).
The company Google perfectly understands that the web designer cannot control entering external links. The strict control is conducted under the proceeding links. You should show consideration for sites on which you refer to. Every possible "suspicious", "black" (according to Google) links on a site cannot harm it, but links from a site can do.
It is necessary to notice also that PageRank is a parameter concerning each separate page but not to all site as a whole. Pages with different PageRank can be on one site.