You decide to build a site or start blogging and you throw some AdSense ads to try to cover some of your costs and time. You're not trying to make it a million dollar business, but you're happy to maybe get a six-pack once in a while for your efforts.
When you started, you weren't sure if you'd make any money with AdSense, but you see that you're doing better than you expected. Google doesn't pay you until you reach $100, and that might take you a while at the rate you're going. So you just churn out your pages and promote your site and wait for the check to come. Maybe you decide to put in a little extra time because of how much you're making.
Unfortunately, just after you clear that $100 mark, you get an email from Google telling you they're disabling your account. You're not sure why, and the email doesn't really clear it up.
Why does that happen?
Google tries to automate everything
The way Google makes money is by automating a lot of what goes on in their systems, including AdSense. They have crawlers and algorithms that go through and interpret data and make decisions automatically based on that information. If they didn't do that, AdSense wouldn't be profitable. Just imagine having to take the time to constantly review the millions of publishers that are using AdSense and the billions of pages they have.
So Google does their best to have their computer programs detect any AdSense Policy violations. When it comes time to pay out, they might give the page a little more scrutiny and possibly have a real person review the account. Or they choose to manually review after a certain number of clicks. It's not clear, but I think it's safe to say they don't hire people to view all the sites in their network.
Since many people don't earn enough to get paid by AdSense, they can cut out a lot of sites, and pay more attention to the sites that are actually being paid.
So it's important to remember that you shouldn't count on your money from AdSense, or any other online program until it's actually deposited in your bank.
Until you start getting paid, don't assume you're doing everything OK, or if you're a Black Hat, don't think you've gotten away with it. You should go through the AdSense Policies every once in a while and make sure you're abiding by them. Also review this post on common AdSense violations on JenSense.
What happens to the money?
According to Google, they don't keep the money. Depending on the reason for the AdSense ban, they may still send out the money that has been accruing in the account, provided it's more than $10. If they suspect invalid clicks, or possibly some other types of cheating, the money will be returned to the advertisers.
If Google did decide to cancel all the accounts and keep all the unpaid money, as of the last reported quarter, Google was sitting on over $500 million in unpaid AdSense funds. Tempting, but they generate almost 4 times that each quarter.
How to get your AdSense Account back?
If you're account has already been disabled, you can try to appeal. First go over the AdSense Policies, linked above, review the Disabled Account FAQ, these pages on Invalid Clicks then go to the AdSense Appeal Form and hope for the best. Try and provide as much information as you can but be brief and to the point. No need to whine about how you need the money and that you're a good person.