PR Is created by backlinks. That's what the algorithm is totally based on. The original Page Rank developed by Larry Page was the primary basis for the Google SE when it was first created.
The SE algorithm has been refined over time and PR is no longer the primary factor used by Google, but it's still very important in determining the value of the links pointing to your site. The actual PR algorithms are still Top Secret within Google, but you can get a good understanding of how it works by looking at the original published Page Rank algorithm.
You can find the original algorithm on wikipedia
http://en.wikipedia.org/wiki/PageRank
and if you search google you can get a lot more information, not only about the original algorithm but changes that SEO experts have determined based on experimentation. This is a good idea so you can understand why your doing what everyone is recommending.
Unless you're planning to sell your site, or you want to sell advertising on your site, PR isn't anything you really need to worry about for your own site. But you do want to track down high PR on other sites. If you want good ranking in the SERPS then you'll get the best results if you have backlinks from those high PR sites.
1. The most valuable would be high PR sites with relevant content.
2. Next most valuable would be either random high PR sites, or low PR sites with relevant content (everyone doesn't agree which is better).
3. And last would be random low PR sites that aren't relevant.
If you have numerous sites under your control, then gaining PR for your own pages becomes more of a goal, because you can then use those high PR sites for backlinks to your other projects.
The main thing you need to remember regarding PR, is that you want as many backlinks from sites that are relevant to your own, and make as many of those be from high PR sites. If you concentrate on that you'll definately see better results than just randomly blasting out backlinks.
This is just a basic guideline which leaves out other important factors such as follow tags. You'll want to learn more about that as well to get the best backlinking results.
Hope this helps explain things a little bit for you.
Wolf