This is probably a good place to begin reading.

A comment from David Smeaton on the G+ site:

bitcoin is interesting because of its anonymity. quite famously, wikileaks started accepting bitcoin partly to protect its donors' identities and partly because paypal shut down their account. bitcoin is also the preferred payment method of tor communities. within tor is a black market site called 'silk road'. they use bitcoin almost exclusively for trading drugs, weapons, and other illegal wares.