On some occasions, Google AdSense is not able to find relative advertisements for your web page. In this case, AdSense randomly chooses a Public Service Ad (PSA) to put in your ad space. Unfortunately, PSA's generate no revenue for you. Google has created the ability to load an alternate ad when it cannot find a matching ad. This is accomplished by setting a variable called google_alternate_ad_url in your AdSense layout code:
google_alternate_ad_url = "/adsense-alternate-ad.shtml";
This type of alternate ad can include advertisements from Google AdSense competitors, such as Clicksor. Using this method enables you to create revenue which you would normally lose due to AdSense PSAs.