AdSense is an affiliate program that pays you when a visitors to your site clicks on the ad displayed on your page. From personal experience, as well as thousands of other publishers, claim AdSense to be the most profitable affilaite system ever.

Since AdSense is able to display relative to each page advertising, the CTR (click through rate) is much higher, which results in bigger commissions.

The lowest cost of 1 click I've experienced was $0.27 - the highest - $14. It depends on the category (content of your site) and competition level of the field.

Getting in AdSense is NOT very hard. If you have a great site, great content - you have big chances of being accepted.

Best,