Adsense is a contextual advertisement program run by Google, and I am sure you've seen "Ads by Google" on a variety of websites (this is Adsense in action). To get enlisted in AdSense is actually simpler that you might think. What you need to do is to create a blog (or site), publish some content there (no junk please) and apply for AdSense.
Once approved you can start making money without having to do too much - just paste code that Google gives you into your web properties. This code creates ads on your blog and when someone clicks on these ads they go to the advertiser's site. The advertiser is then charged per click by Google and Google pays you a share of the money. As easy as that, and all done for you as part of Adsense service.
I mentioned "contextual advertisement", what it means is that AdSense is smart enough to deliver ads that are relevant to your content - automatically. Let's say you have a blog about dog training, you'll see ads about dog training on it. Google technology analyzes your content, decides what it is about and then finds the ads that are most suitable for your blog content.
By making AdSense ads relevant to your site, Google accomplishes 2 things: ads can enhance overall experience of a visitor to your blog, and because ads are relevant to your content and visitor, they are likely to get clicked on.
The best part is you'll get paid and make money every time ads get clicked on.
Max Lloyd is internet entrepreneur, writes about his experiences in making money online and publishes Viliat Online Business blog at http://www.viliat.com