You can past code onto your website to show Google Ads, or any other ad network. Determining which will be most profitable for your site is next to impossible. Therefore let someone else do it for you automatically. PubMatic does just that, they determine which ad network is the most profitable for any advertisement size you choose.
PubMatic aggregates ad networks and optimizes to provide your site with the best performing ads. PubMatic allows you to place code on your site once for a particular size and receive the benefit of multiple ad networks filling that advertising space.
You simply join PubMatic for free and then link the account using your names and passwords to other ad networks such as Google AdSense, ValueClick, Advertising.com, BlueLithium, and others. You then place code on your site for your advertisements, standard sizes such has 300×250 728×90 and more, just as you would for any of the ad networks above. The difference is that you are placing PubMatic code and PubMatic will serve ads from all the ad networks you are a member of.
PubMatic will then optimize by placing the ads from the best performing network. Awesome technology and wonderfully executed. I have created an account and am testing their technology on this site. I find their interface to be easy to use, intuitive, and was up and running in minutes.