A virtual currency, bitcoins do not rely on a central bank or financial institution. They're based on an open-source protocol and "mined" on computer servers. When the total number of bitcoins reaches 21 million, no more coins will be mined. Currently, there are about 11.7 million bitcoins in circulation, valued at about $138 each -- an amount that fluctuates.
That's a lot of virtual dough. And unlike credit cards or even Paypal transactions, bitcoins are not tied to a particular person, which makes them ideal for buying products or services that might be embarrassing or illegal.
In fact, black marketeers used bitcoins on the underground website Silk Road, which, until it was shut down by the FBI on Oct. 1, made up most of bitcoin transactions. According to the FBI, the number of bitcoins used on Silk Road between February 2011 and July 2013 came to 9.5 million, or $1.3 billion. Those sales generated some $79.8 million in commissions for the site. On the same day the site was shut down, the FBI also arrested its owner and proprietor, Ross William Ulbricht.