BBC NEWS | Technology | Rings of steel combat net attacks
Gangs demand protection money from gambling sites. If they don't pay up, they are hit with a denial of service attack.
The interesting part is that they don't seem to target legitimate businesses (Amazon), or at least the journalist didn't mention that. I suspect this is simple logic. Amazon is unlikely to pay up, and they're a much tougher target. It's the same reason physical world protection rackets don't target Walmart.