Admob?

I don't know anything about Admob. I don't know what it is or why it's used. Is there documentation I can read on this subject. I'm assuming it's ads that you get paid for if your game is downloaded? Not sure about it. Is it something I should be using? Any help would be great.
Thank you..

here is a link is very good about AdMob

http://www.google.co.ve/ads/admob/

Thank you!

Just a question. When a user clicks on an Ad and the site opens, how do they get back to the game or App. When you hit the back button the app closes when I test this. How can they get back to playing the game? Are there events I'm missing??

tested here an works, the back button is suspended during this exit.

but if you want to be sure, uncheck "End when not in foreground" and use the android object to exit when you want. (detecting the back button and ending the application)