AdSense is an application created by Google that enables Webmasters to advertise  on their websites easily while earning a percentage of the ad revenue. Each time somebody visits your website and clicks on one of the ads provided by Google, you receive anywhere from a few cents to a few dollars per click.

Here is how it works:
1. You sign up for an account at Google AdSense.
2. Wait for Google to approve your account. You must have an active website for them to do this.
3.  They provide a code that you can install wherever u want on your web pages.
4.  Google crawls your page for common keywords and uses these to determine what kind of content should be advertised on you site.

Certain keywords yield more lucrative advertisements. Ads about insurance, loans, and mortgages tend to provide the highest pay out when people click them on your page. If you involve keywords on your website that attract these kind of ads they can create a much larger financial return in the long run.

Google AdSense is in no way a get rich quick scheme. But if you follow all of their rules, and generate enough web traffic, it can become a lucrative way to gain a little extra on the side.