This paper identified and understanding of crypto currencies including bit coin and the block chain technology. Crypto currency is an encrypted, decentralized digital currency transferred between peer to peer networks and confirmed in a public ledger via process known as Mining. The public ledger is called Block chain and every process along the way is secured by cryptography. Crypto currencies require strong,secure mining algorithm. Crypto currency transactions are sent between peers using software called crypto currency wallet. These allowing users to reach secure, robust consensus for each transaction. Crpto currency roughly equalent to Paypal or Debitcard. Bit coin is the first and most popular cryptocurrency, developed in 2009 by name of Satoshi Nakamoto. A poorly peer to peer version of electronic cash would allow online payments to be sent directly from one party to another. Main features of these currencies are payments can be made without the involvements of banks, and other banking charges. A digital signature provides part of the solution and to prevent double spending problem. One can use this technology for many transactions in devices, like mobile phones and tablets etc.