I will leave this to Mr. WikiPedia:

AdSense is an ad serving program run by Google. Website owners can enroll in this program to enable text, image and, more recently, video advertisements on their sites. These ads are administered by Google and generate revenue on either a per-click or per-thousand-impressions basis.

If you notice, which I am sure, you will see the below advertisement in most of websites on the Internet.. and yes, these ads are from Adsense. They are everywhere!

bet you tried clicking it.. you clickerss!

Before you sign up as a Adsense Publisher, please read through their Terms and Conditions and their Program Policies and make sure that you don’t break any of the rules. A lot of people missed that and end up being banned by Google. When you are banned, it is for life!