Google Page Rank (or PR as it is often referred to as) is simply an indication of the number of websites that link to a specific website. It also attempts to indicate the quality of those links. PR ranges from 0 to 10 (with 10 being the 'best' PR and 0 being the 'worst'). The vast majority of small business websites will usually find they have a PR of between 0 and 5.
To calculate a particular sites PR, Google uses a fairly complicated algorithm based on the number of web links that it is aware of that link to the site in question. This algorithm will also take into account the PR of the page that is providing the link, thus a link from a web page that has a PR of 7 will be considered more valuable than a link from a page with a PR of 4.
Because of the way in which links from higher PR-ranked sites are considered more important, many people are choosing to buy links from websites with high PR's just so that they can increase their own PR. I have seen sites selling a simple text link on their home page for over $700 a month purely based on the fact that they have a PR of 7 or above. This may seem like a lot of money but when you consider that the website owners that are buying these links often have websites that are in no way relevant to the content of the site linking to them, it is absolutely ridiculous.
Take this example, let's say you have a website about health and fitness and you buy a link for $500 a month from a random website because it has a PR of 7. This random website has no relevance to your health and fitness site so what is going to happen? Well, your own PR may increase as a result of the link. You may get a bit of extra traffic but probably not much since people don't click on links that that they are not interested in. You will definitely be $500 poorer at the end of the month!
Instead, why not spend the $500 on pay-per-click advertising and benefit from some quality, targeted traffic?
Of course, there is a bit more to it than that and the reason that most people want to increase their PR is because Google takes this statistic into account when determining where a website will be displayed in their search results. Many people assume that a high PR automatically equals a high search engine placement for their chosen keywords. Not so....
PR is just one of over 100 different factors that Google takes into account when deciding where your website will feature (and these factors and the main algorithm change on a very regular basis). It is perfectly possible for a website with a PR of 5 to get a higher ranking than a PR 7 site if it has better content or is more relevant for the search term in question.
Remember that relevance is all important with Google and a link from a website that is not relevant to your own site will be considered far less important than a relevant one (which makes buying links from random sites purely because they have a high PR even crazier).
I have read several rumours lately that Google haven't updated PR's for a couple of months and they are considering phasing PR out or modifying it in some way. This is pure speculation but it wouldn't surprise me in the least. PR is easily manipulated (for example by purchasing links as described above) and Google doesn't like to have their calculations or search results manipulated. It stands to reason that they will be looking at ways of preventing this.
Well, it is a good indicator of how many other sites link to yours and how important Google considers your site to be BUT I personally don't place too much importance on this statistic and I certainly won't be paying out for a link from a website just because it has a high PR.
As I said above, Google changes it's rules on a regular basis and I see little point in chasing a particular PR on the basis that it might get you higher search engine rankings. If Google do decide to do away with PR, all your work will have been for nothing.
Instead, concentrate on building quality, relevant links from sites that are connected in some way to your own site content. This will ensure that any traffic you receive via these links will at least have an interest in your site. Building links on this basis will automatically increase your PR over time (without the need to pay out for overpriced, irrelevant links). If you do things this way and Google does scrap the PR indicator, it shouldn't affect you in any way and the links you have in place will continue to benefit you.
Remember a High Page Rank doesn't guarantee traffic or sales. Sure a high PR is a 'nice to have' but lots of traffic and high sales is even nicer