Yeah Domenico is right. Another term for it is money laundering. Basically say you've earned $100,000 illigally through drugs etc when your legal income is about $20,000 a year. There is no way you could put this in your bank account because you'd be investigated - it looks so dodgy. Especially with the tax man who wants a % of everything you earn.

So, what you do is clean it. This can be done by putting the money in overseas accounts where it cannot be traced. Then you can get it transfered back to you and like Domenico said, put the money into legal operations. With the money in legal operations, say a new business, your 'legal' income would now be higher than the $20,000 originally , say now to $40,000. Which means you can 'legally' afford your new larger house without the government getting on your back.