Google runs an ad serving program called Google AdSense. Website owners can enroll in this program to enable text, image and, more recently, video advertisements on their sites. It is a fast and easy to display relevant Google ads on the website's content pages.

These ads are administered by Google and generate revenue on either a per-click or per-thousand-impressions basis. If used properly, it can generate a very large and healthy income for them.

It's also a way for website publishers to provide Google web and site search to their visitors, and to earn money by displaying Google ads on the search results pages.

YPN Yahoo! Publisher Network:

You can easily make money online with Yahoo! Publisher Network. It is a beta program through which Yahoo provides text ads in much the same way that Google does. Publishers of all sizes have access to products and services designed to earn money.