In the continuing improvement of its Webmaster Tools, Google has added a new "URL parameters" section under the "Site configuration" drop down menu.
This is an important feature in helping webmasters control the Google's crawl rate of sites where the displayed content can be modified or filtered based on URL calling parameters. For example, this can be particularly important to online merchants and stores.
As best as we have determined, Google follows nearly every hit on your site with their own mirrored hit. It records all the parameters and then has its Googlebot crawl your pages with the new parameters. By doing this, Google can index your various pages based on your content. For example, let's assume that you own an online furniture store, called onlinestoreexample.com.
When someone visits your store at that url, they are shown a series of departments. Let's say Department 1 is bedding and Department 2 is carpets. If your visitor clicks on the bedding link, they will access the page using the url: onlinestoreexample.com?dept=1. If they want to visit the carpet section, they click on the url onlinestoreexample.com?dept=2.
This is all well and good until someone tries to access your pages by typing the url themselves. If they make a mistake entering the dept parameter and type in "dpt" or "..." instead, then these parameters will be passed along to the Googlebot crawler. This means that your site will be crawled much more often than necessary and that the error pages you display become indexed as well.
By now enhancing Webmaster Tools and giving webmasters the ability to see the crawling parameters, one can now inform Google whether the parameters that they use for crawling are valid or not. For each parameter listed, you have a choice of letting Google decide if it is important, or if it does not affect your page content.
In our own website, we were shown 19 parameters. Of those, 7 were invalidly formed and resulted in errors. By indicating that these 7 parameters did not affect our content, approximately 400,000 urls were eliminated from the crawler.
So, if you have a website and rely on Google Webmaster Tools, we recommend that you visit your own URL parameters section and learn how Google sees your site.