I think the name of this article gives the best definition that could be found for the AdSense code of Google.

Through its AdSense program Google let you earn without the positive need for you to sell anything at all to anyone anywhere.

Many persons, including who is writing this article right now, are generating money and receiving checks from Google month after month.

And there are many internet entrepreneurs that are building higher incomes from participating in the AdSense code than from the direct sales of their own products or services.

All right, you are thinking that I told you something that simply has no meaning as earning funds without selling.
How may it work?

The answer is simple, you are paid per click and not per sale.

The AdSense program of Google is a service that let you, and anyone else that provides some kind of content online, to become a publisher for the Pay Per Click branding script they run.

You might see it as a triangle, where three different subjects are involved:

- Advertisers
- Google
- Publishers

The advertisers join Google PPC service (named AdWords) in order to promote their services or products to the huge audience that this giant search engine has each day.

They bid on targeted keywords that are relevant to the services or products they sell and have their ads displayed on Google when searches are made for the chosen keywords.
They pay to Google for every visit to their sites that comes from this promotion, so they pay for each click.

Google then opened another accomadation called AdSense that is an addition to AdWords.
It is a accomadation for online publishers, and let you display on your web pages ads of the AdWords advertisers explicit to the keywords of your web pages.
It is all done automatically when you add the simple code that AdSense gives you once you are approved for their accomadation .

Once a visitor of your sites clicks on one link of the AdSense ads that your website displays the AdWords advertiser is charged by Google for the click, or visitor that is delivered to his page , and you are credited by Google a share of that funds in your AdSense account.

So you are paid for selling nothing at all, but just for the click that the AdSense ad received in one of your web pages.

Of course the Google AdWords advertisers are very interested in selling the products and services they promote paying for every visitor sent to their page , and I sincerely hope that the clicks that increase my AdSense income benefit the advertisers that pay me through Google, but this anyway is not my concern or something I have to worry when I create my sites and add the AdSense code to them.

I am more interested in making web pages that are interesting for my visitors and provide them useful content.
I worry to provide them with explanations and links of workable resources if I do not have directly the opportunity to offer them a merchandise or a accomadation , and the ads displayed by Google are an addition that could be interesting and useful for them and may increase my income.

I think it seems all very simple from the above explanations.
One thing that is very important to understand is that the Google AdWords program is extremely crowded and there are really a lot of keywords and key phrases with big competition and very high bids.
This means that advertisers pay a lot of money for every single click for those keywords.

Of course also AdSense publishers can earn a lot of money if they have sites related to those keywords that receive many visitors and many clicks to the Google ads.