Link popularity (or the number of pages linking to another page) is one of the key methods search engines use to determine the importance of a website. All search engine use some form of link popularity as part of the way they rank sites. Google have trademarked the name PageRank (PR) as their way of measuring link popularity. They have also patented the process. The PR assigned to a page (not a site) is a measure of:

To get the PR of a page, install the Google toolbar and join the obsessions of watching the little green bar (the PR will need to be set up to display via the options menu). PR can also be obtained by looking at the green bar beside the site in the Google Directory, if the site is listed there. The tool below can also be used.

Google periodically update the toolbar both internally for their own use in the ranking algorithm and externally or publicly via the toolbar and in the Directory - all these updates are on a different and variable timeframe. When the PR is publicly changed on the toolbar and in the Directory, the search results do not change (they used to in the past) as it is widely believed that, that PR has already been factored into the search results internally before the public update (some have suggested as long as 2 weeks before).

Some of these dates may vary by a day or so from other lists - it all depends on when you call the update. When it starts? When its finished? Which part of the global time zone you are in? Google have a number of data centers that updates need to occur at, so this process of the toolbar update takes from a day to up to almost a week.

When a public PR update is happening, this tool from SEOChat can be used to check a pages PR at the different data centers:

As PR is a mathematical formula of incoming links to a page, the only way to increase PR is to get more links. But, its important that the relationship of PR to the ranking of a page (they are different things) need to be kept in perspective. Also, it may take a while for Google to crawl or spider a new link to a page, it may even take months to do an 'analysis' of the link before it gives any credit for it, or it may even sandbox a link for 6 or so months.

It is widely believed that in the mathematical calculations of PR, that the amount of PR available for the whole web is a finite number. As the web grows (which it is at an exponential rate), then the amount of PR naturally available for each individual page, will decrease. When PR is updated on the toolbar, all SEO forums have threads on it with a fairly typical pattern - those with new pages go from 0 to a higher value (they are happy). Some have a PR decrease and some have an increase. Those webmasters with a decrease often express anger toward Google for doing it to them! If no links to a site were gained since the last update and the PR of incoming links stayed the same, then all things being equal, there will be a decrease in PR as the web is growing. To avoid the decrease, more links need to be gained (just to retain the same PR). TO get an increase, even more inks would be needed.

Having said that, all this is largely 'academic' as PR is only part of the ranking formula and the downward pressure on PR at updates would be affecting a webmasters competitors as well. So what is important is, how is the ranking of the page?