A cryptocurrency (or crypto currency) is a medium of exchange using cryptography to secure the transactions and to control the creation of new units. Cryptocurrencies are a subset of alternative currencies, or specifically of digital currencies. Bitcoin became the first decentralized cryptocurrency in 2009. Since then, numerous cryptocurrencies have been created. These are frequently called altcoins, as contraction of Bitcoin alternative. Cryptocurrencies typically feature decentralized control (as opposed to a centralized electronic money system, such as PayPal) and a public ledger (such as bitcoin’s block chain) which records transactions.