It’s being called the largest online-money laundering prosecution in history. U.S. officials have accused Liberty Reserve of moving $6 billion for criminals involved in credit card fraud and Ponzi schemes by converting cash to virtual currency.

But before we get to the nitty gritty of today’s sting, let’s back up and explain money laundering.

Here’s a scene from AMC series “Breaking Bad” where a young, upcoming drug dealer is being schooled on the perils of making fistfuls of cash.

The easiest way for the tax man or law enforcement to nab you is to follow the money, says David Beim, a professor at the Columbia Business School.

“Money laundering is an effort to complicate and confuse and obscure the trail of money,” Beim says.

But criminals can’t park their money in banks, after all, banks are regulated by the government. And so it appears, criminals are turning to virtual currencies — which brings us to Liberty Reserve. The government is alleging that criminals exchanged their cash for virtual money through Liberty Reserve, which didn’t ask any questions, making the money virtually untraceable.