AdSense is a pay per click ( PPC ) advertising program run by Google. After they accept your site for AdSense program they give you ad codes to put in your websites. You can login to your adsense account and get these codes.

When a visitor who comes to your site click on one of these ads you get paid. Click value is not same but change depending on your keywords. if your keyword is "Web Hosting" your clicks might be around $1-$5 or if its about "movies" then it might be $0.02-$0.20. How much you earn from your site would be depends on keywords of your site and your traffic. You get paid for each click visitors made. This is the most popular way of earning money among web site owners. Google AdSense is the best way to earn money online because they are the best paying PPC adnetwork today.