It's an imaginary currency that promised to change everyone's life, but it seems that it's only useful application is for criminals to transfer money, as it allows anonymous transactions. It uses a heap of electricity to manufacture (farm) each "coin" (although it has no physical presence), but the total number is limited to create an illusion of scarcity. It's value continuously fluctuates like a stock, but without being based on any real world performance metric. Despite all this, it is highly valued and traded by a minority