Crypto currencies are one type of digital currencies that works as a layer/media to exchange that uses cryptography to prevent and secure the online transcations.The control of crypto currency works through a public transaction database that is called as block chain.
Crptogrphy used for the continous monitoring of the coins that is provided by block chain. Bitcoin was firt developed and after that several cryptocurrencies have been developed and they are called as alternative coins.
Block chain records the transaction between the parties in a smooth way and that cannot be altered without the alteration of all related blocks. The various coins used as a crypto currencies include Bitcoin, Ripple, Ethereum etc..