Google's AdSense is perhaps the fastest and easiest way to generate revenue from a website that contains great content.
If you'd like to see an example of Google AdSense in action, visit this page about what is the best day to purchase a cars. On the left side of the page, you'll see "Ads by Google" followed by one to five text-based ads. The ads themselves are called "Google AdWords," and the program that lets publishers display them on their site is called "Google AdSense."
Here is how Google answers the question -- what is AdSense -- in its FAQ (frequently asked questions):
" Google AdSense is a fast and easy way for website publishers of all sizes to display relevant, un-obtrusive Google ads on their website's content pages and earn money. Because the ads are related to what your users are looking for on your site, you'll finally have a way to both monetize and enhance your content pages.
"The program is free, and combines pay-per-click and pay-per-impression advertising - meaning you get paid for valid clicks on the ads on your site or search results pages as well as impressions on your content pages. So go ahead and try this program.
What is AdSense: A Brief History
Google AdSense is an outrgrowth of the Google Content-Targeted Advertising program that was launched in early March 2003.
This early program let large websites integrate Google AdWords into their websites. Each deal was independently negotiated with Google, and sites with less than 20 million page views couldn't participate.
The Google AdSense program lets much smaller websites play. Even if your site only gets a few hundred page views per month, you can now apply to the AdSense program.