Many publishers don't sell all of their ad inventory. In these situations, publishers might not serve any ads or might serve less valuable house adsto unsold ad inventory, therefore losing potential earnings. Google AdSense is integrated with Google Ad Manager so publishers will always have an ad to serve in an undersold situation.
AdSense is also able to compete in real time on price with your non-guaranteed inventory to make sure you're always showing the most valuable ads. If AdSense can pay you more than the alternative, an AdSense ad will show. When that's not the case, an ad from your highest paying non-guaranteed inventory will be shown.
Google will always serve the highest paying AdSense ad available, and will never lower the price of the winning ad, or reduce your earnings from it.
How is Google AdSense different than Google Ad Manager?
With Google AdSense, you can display contextually-relevant, unobtrusive Google ads on your website's pages to earn money.
With Google Ad Manager, you can manage your directly-sold and network-based online advertising inventory in addition to Google AdSense.
Can I sign up if I am already a part of an ad network?
Yes, although we do have some guidelines as to how non-Google ads may be displayed along with Google ads. We do allow affiliate or limited-text links.
If your ad network already displays Google ads on your pages, you can still join AdSense and run ads on your site through our program. We allow publishers to display up to three standard Google ad units on any webpage.
Keep in mind that it's your responsibility to work out any contractual issues that may arise with your ad network as a result of serving Google ads directly.