what is cryptocurrency

A cryptocurrency (or crypto currency) is a digital asset designed to work as a medium of exchange using cryptography to secure the transactions and to control the creation of additional units of the currency.[1][2] Cryptocurrencies are classified as a subset of digital currencies and are also classified as a subset of alternative currencies and virtual currencies. A cryptocurrency wallet stores the public and private keys which can be used to receive or spend the cryptocurrency.

Bitcoin became the first decentralized cryptocurrency in 2009.
Since then, numerous cryptocurrencies have been created.

These are frequently called altcoins, as a blend of bitcoin alternative.
Bitcoin and its derivatives use decentralized control as opposed to centralized electronic money/centralized banking systems.The decentralized control is related to the use of bitcoin's blockchain transaction database in the role of a distributed ledger