Page Rank is Googles way of showing the user a basic ranking of a webpage. The ranking is from 0 to 10 with 10 being the highest. To calculate page rank there are numerous variables involved but the most important in link popularity. So the more incoming links you have to that webpage the higher Page Rank you will have.
Page Rank is updated all the time but not visible to users. The visible Page Rank on the Google toolbar is only updated a few times a year. Normally every 3 months. There are a few ways that you can get a general idea of your Page Rank at all times. One way requires the site to be listed in the Google directory. It has become known that the directory lists in order of Page Rank. The directory uses current Page Rank data and not the visible one that only updates every 3 months. So if you have a site in the Google directory you can always get a general idea on what your current Page Rank is.
There are also Page Rank predictor tools out there. Now these tools used to be somewhat accurate but they have been shutdown with Google changing the PK parameters to all 0s. This may change again in the future but as of right now it simply does not work.
One thing that gets me is how Page Rank consumes people that are new to the SEO world. To get a few things straight here is what Page Rank does for your. NOTHING! You do not rank higher in the SERPs, make more money, get more traffic, or look cooler. This number is just a basic guideline of how many incoming links you have to your site.
But Page Rank is good for a few things at the same time. Most webmasters base link exchanges primarily based off of Page Rank. So the higher Page Rank you have the better chance of getting a good link exchange. Also it is good for selling links on your site. The price pretty much always reflects what the Page Rank is on the page.