A cryptocurrency is a digital or virtual currency that is secured by cryptography, which makes it nearly impossible to counterfeit or double-spend. Cryptocurrencies are decentralized networks based on blockchain technology—a distributed ledger enforced by a disparate network of computers. Cryptocurrencies are a form of alternative currency and digital currency (of which virtual currency is a subset). Cryptocurrencies use decentralized control as opposed to centralized digital currency and central banking systems.