A cryptocurrency is a digital asset designed to work as a medium of exchange that uses strong cryptography to secure financial transactions, control the creation of additional units, and verify the transfer of assets. Cryptocurrencies use decentralised control as opposed to centralised digital currency and central banking systems.
The decentralised control of each cryptocurrency works through distributed ledger technology, typically a blockchain, that serves as a public financial transaction database.
Bitcoin, first released as open-source software in 2009, is generally considered the first decentralized cryptocurrency. Since the release of bitcoin, over 4,000 altcoins (alternative variants of bitcoin, or other cryptocurrencies) have been created.