Let's use the wikipedia definition here, since I like it :
"A cryptocurrency (or crypto currency) is 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. Cryptocurrency is a kind of digital currency, virtual currency or alternative currency. Cryptocurrencies use decentralized control as opposed to centralized electronic money and central banking systems. The decentralized control of each cryptocurrency works through distributed ledger technology, typically a blockchain, that serves as a public financial transaction database."
The first cryptocurrency ever started was the Bitcoin and it still remains the biggest of them all. BTC was invented by Satoshi Nakamoto, a mystery man that pretty much nobody knows anything about.