A cryptocurrency is a digital or virtual currency that uses cryptography for security and is decentralized, meaning it is not controlled by any government or institution. Cryptocurrencies are based on blockchain technology, which allows transactions to be recorded and tracked securely and transparently on a decentralized ledger. Cryptocurrencies use complex mathematical algorithms to verify and secure transactions, and to create new units of the currency. They are often used as a medium of exchange for goods and services, and can be bought, sold, and traded on online exchanges. Some popular examples of cryptocurrencies include Bitcoin, Ethereum, and Litecoin.