Bitcoin is the first, and the biggest, "cryptocurrency" – a decentralised tradable digital asset. Whether it's a bad investment is the big question . Bitcoin can only be used as a medium of exchange and in practice has been far more important for the dark economy than it has for most legitimate uses. The lack of any central authority makes bitcoin remarkably resilient to censorship, corruption – or regulation. That means it has attracted a range of backers, from libertarian monetarists who enjoy the idea of a currency with no inflation and no central bank, to drug dealers who like the fact that it's hard (but not impossible) to trace a bitcoin transaction back to a physical person.
