A crypto currency is a medium of exchange using cryptography to secure the transactions and to control the creation of new units. Crypto currencies are a subset of alternative currencies or specifically of digital currencies. Recently “Bitcoin” is used as crypto currency. Bitcoin was invented by Satoshi Nakamoto. It was implemented as open source code and released in January 2009. Bitcoin is often called the first crypto currency although prior proposals existed. No one controls it. However, bitcoin’s most important characteristic, and the thing that makes it different to conventional money, is that it is decentralized. No single institution controls the bitcoin network. In this study we try to explain that, what is it?, is it legal?, how’s it work?, is it has got enough security?