A digital or cryptocurrency is a form of currency that is available only in digital or electronic form, and not in physical form which typically utilize strong cryptography to secure financial transactions, control the creation of additional units, and verify the transfer of assets. The decentralized nature of these assets means that distributed ledger technology is used, typically a blockchain, which acts as a database for transactions.
Bitcoin and Altcoins
Bitcoin, first released as open-source software in 2009, is considered the first decentralized cryptocurrency. Since the release of bitcoin, thousands of altcoins(alternative variants of other cryptocurrencies) have been conceived. You can view a list of them in the knowledge base here.