PageRank is Google's method of measuring a page's "importance." When all other factors such as Title tag and keywords are taken into account, Google uses PageRank to adjust results so that sites that are deemed more "important" will move up in the results page of a user's search accordingly.
 
A basic overview of how Google ranks pages in their search engine results pages (SERPS) follows:

1. Find all pages matching the keywords of the search.
2. Rank accordingly using "on the page factors" such as keywords.
3. Calculate in the inbound anchor text.
4. Adjust the results by PageRank scores.

In reality, it's slightly more complex and we'll discuss this in more depth later, but for now the above description serves our purposes. It's worth noting that PageRank is a multiplier and is not just simply added to the score. Thus, if your page had a PageRank of zero, it would rank at the very end of the SERPS.
 
How is PageRank determined?
The Google theory goes that if Page A links to Page B, then Page A is saying that Page B is an important page. PageRank also factors in the importance of the links pointing to a page. If a page has important links pointing to it, then its links to other pages also become important. The actual text of the link is irrelevant when discussing PageRank.
 
How can you tell what a page's PageRank is?
To learn what a page's PageRank is, you can download a toolbar for Internet Explorer from http://toolbar.google.com. Once installed, there will be a bar graph at the top of the browser showing a version of PageRank for the page you're browsing. When you hold the mouse over the bar, you see a number from zero to ten. (If you don't see the number, you may have an older version of the toolbar installed. You will need to completely uninstall it, reboot your computer and reinstall the latest version. Once this is done, you should be able to see the PageRank number.)
 
How accurate is the Google toolbar?
The Google toolbar is not very accurate in showing you the actual PageRank of a site, but it's the only thing right now that can give you any idea. As long as you know the toolbar's limitations, then at least you know what you are viewing. There are two limitations to the Google toolbar:

1. The toolbar sometimes guesses. If you enter a page, which is not in its index, but where there is a page that is very close to it in Google's index, then it will provide a guesstimate of the PageRank. This guesstimate is worthless for our purposes because it isn't featured in any of the PageRank calculations. The only way to tell if the toolbar is a guesstimate is to type the URL into the Google search box and see if the page shows up in the SERPS. If it doesn't, then the toolbar is guessing!
    
2. The toolbar is just a representation of actual PageRank. Whilst PageRank is linear, Google has chosen to use a non-linear graph to portray it. So on the toolbar, to move from a PageRank of 2 to a PageRank of 3 takes less of an increase than to move from a PageRank of 3 to a PageRank of 4. A comparison table best illustrates this phenomenon. The actual figures are kept secret so we'll just use any figures for demonstration purposes:
   

The PageRank shown in the Google directory (http://directory.google.com) suffers from the same problems. The PageRank shown in the directory is also on a different scale. There have been attempts to cross-reference these two scales but because they are non-linear, the results really do not tell you anything more than you already know.
 
Also of note is that a programmer managed to generate a tool to look up PageRank without using Internet Explorer. This tool has since been withdrawn, but whilst originally the numbers given by this software and Google's toolbar matched - presently querying with such software sometimes produces different numbers than querying with the toolbar. This is Google's right to protect their data, but is the strongest indication that:
 
Sometimes what you see on the Google toolbar may not be related to actual PageRank at all. (Google can and does, assign whatever toolbar PageRank value they want to assign to a page.)
 
Is PageRank a good determination of the quality of a page?
To examine the worth of PageRank, we need to first look at its premise, and how accurate it is. Basically PageRank says:

1. If a page links to another page, it is casting a vote, which indicates that the other page is good.
    
2. If lots of pages link to a page, then it has more votes and its worth should be higher.
    
   The basic implication here is: People only link to pages they think are good. It shouldn't be hard to convince you that this premise is wrong. A few of the reasons people link to pages other than ones they think are good are:
    
   • Reciprocal links - "Link to me and I'll link to you."
   • Link Requirements - "Using our script requires you to put a link to our page." or "We'll give you an award solely because you link to our page."
   • Friends and Family - "This is my friend Pete's site." or "My mum's site is here, my dad's site is here. My dog's site is here."
   • Free Page Add-ons - "This counter was provided by www.linktocountersite.com."
   
   

Furthermore, anybody who has a top-ranking site will tell you that it tends to get links from new sites. This is not necessarily because it's good (although they generally are). Assume a Webmaster is setting up a new site and they are looking for some outbound links. Nowadays, one of the first things they do is a Google search for similar sites. The links they end up with may not necessarily be the best sites, but merely the easiest ones to find. If PageRank influences rankings, and if they subsequently link to those pages - the new Webmaster will be adding to the inaccuracies in the judging of the quality of a page. The same is true when these new Webmasters use the Google Toolbar PageRank indicator to choose whom to link to.
 
To put this another way:
 
PageRank is determined by the links pointing to a page (Link Popularity). But if PageRank itself has an influence on the number of links to a page, it is influencing (in a circular way) the quality of that page. The links are no longer based solely on human judgement. If a Webmaster picks their outbound links by searching on Google or by looking at the Google toolbar (even if this is only part of their judgement), then there is a corresponding increase in a page's PageRank. This increase is not solely because it is a good page, but because its PageRank is already high.
 

 
* This article was written and theorised by Chris Ridings and Mike Shishigin
 
Chris Ridings is the original author of PageRank Explained. A former programmer of custom search engines, search engine expert, creator of the search engine optimization support forums (http://www.supportforums.org) and author of marketing software tools (http://www.iebar.com/). Chris uses his programming experience and knowledge of algorithms to get an insight into how search engines work "under the cover."
 
Mike Shishigin, PhD, is a brilliant search engine expert, whose philosophy is to always back up all his words with hard facts derived from deep knowledge of applied mathematics. As Director of Technology at Radiocom Ltd. Mike uses methods of sampling analysis to get into the nature of things, providing users of WebSite CEO with an outstanding Web site promotion suite (http://www.websiteceo.com); thus providing them with the best possible results in the search engines. Mike controls all R&D and search engine studies within WebSite CEO. His latest "hobby" is the PageRank technology because it is similar to graph theory, his favourite university course. His strongest dislike is search engine spam, which he considers the deadly sin of Internet age. Mike mercilessly fights it by enlightening minds of WebSite CEO users and making regular raids into spammers' camps.