As an AdSense partner, you know that Google is continually making improvements to the way we match ads to your content so that users see even more relevant ads, which should help you make more money over time.

A "referral URL" is one of many signals we use to deliver contextually relevant ads on your website. The referral URL contains information about the link a user followed to arrive at your website, whether from a search engine or another site on the Internet. Any webmaster for any site can look at referral URLs to see how users arrive at their site.

Let's see how this works today when a user arrives at your golfing advice website from a search engine results page. Imagine that someone searches on Google for [golf shop atlanta] and clicks on a search result that takes them to your site. The referral URL that is passed to your site may look something like this: I'm using Google as an example here, but the same type of information is transmitted if a user arrives at your website from another search engine.

To deliver the most relevant ad, we treat the query words [golf shop atlanta] in the referral URL as if they're part of the content of your webpage. We can then better tailor the ad we deliver on your site. In this example, we could use the additional information from the query words to show an ad for a golf shop in Atlanta rather than for one in Chicago (depending on the other words in the page).

We've recently started to expand the use of the query words in referral URLs to a few hours so we can so we can continue to deliver more relevant ads. The technical way that we're doing this is by associating the relevant query words in the referral URL with the existing advertising cookie on the user's browser. After a short period of time (a few hours) the query words are no longer used for the purposes of matching ads. Of course, users can continue to opt out of our advertising cookie at any time here.

This allows us to deliver more relevant ads on a wider range of AdSense partner sites that a user may browse over the course of a few hours. Let's assume the user in our example leaves your golf website and browses through to a news website that is also an AdSense partner. Since [golf shop atlanta] is in a referral URL that was visited in the past few hours, we may use those query words, along with the content of the news webpage itself, to determine the most relevant ad to show the user on the news website.

Using signals from the referral URL is just one part of our teams' continuing efforts to deliver even better contextually matched ads on your website. 

Update August 2012: Google now uses query words in referral URLs for up to 30 days.