A cryptocurrency is a digital asset designed to work as a medium of exchange using cryptography to secure its transactions and to control the creation of additional units of the currency.
Bitcoin and its derivatives use decentralized control related to the use of blockchain transaction database in the role of a distributed ledger. Decentralization means that the system is peer-to-peer — all transactions take place without an intermediary, between users directly.
Cryptocurrencies are mostly pseudonymous — their owners are not explicitly identified. Each balance is simply associated with an address and its public-private key pair. The money "belongs" to anyone who has the private key and can sign transactions with it.
There are cryptocurrencies that aim for the true anonymity. Amongst them are Monero and Zcash.
As of September 2017, there are over 1100 digital currencies in existence. These are frequently called altcoins (bitcoin alternatives).