Google PR, or PageRank, is a part of Google's ranking algorithm in which every individual page of a site is ranked (aka has its own Page Rank) based on how many other pages on the web link to it, and the authority of those pages (in other words, the quality and quantity of sites linking to each of them).
No one seems to have mentioned it yet so I guess I will ... Toolbar PR is not what it used to be, and is today rarely updated and not a true reflection of the internal PR that is used in the Google algorithm. It's for show only.
/rant/ BTW, it's kind of obnoxious when people keep asking the same questions over and over again. Search before you post, folks ;) There are about a million threads on here asking what PR is. /end rant/
PageRank or PR is the ranking algorithm by Google. There is a Wikipedia article that explains this in detail. PR is computed on the link structure of a website. Computation is based on the number of inbound and outbound links and the quality and relevancy of these links.