they are two different things. Page rank is the measurement of the value of your pages in the eyes of google, and backlink is the text ad of your site on other sites. It is just an advertisement of your product, business or services which you want to popularize among the people. It boosts your search engine position.
PR is just a number nothing more Backlinks are other sites/pages linking to your site/pages Without PR or with small PR 0 1 etc You can Rank and Have Hight Traffic without backlinks no chance but this doesn't mean by building links like an idiot you get on First Page ... is more to SEO then building backlinks
backlinks are a way to boost your website traffic, page rank is just a way of determining your websites importance
If your are referring for toolbar PR, it is something that you can see in toolbar if your have a toolbar that checks for PR. On the other hand, backlinks are links from other sites/pages that direct to your website.