Bitcoin is a peer-to-peer payment system introduced as open source software in 2009. The digital currency created and used in the system is alternatively referred to as a virtual currency, electronic money, or a cryptocurrency because cryptography is used to control its creation and transfer. Conventionally, the capitalized word “Bitcoin” refers to the technology and network, whereas lowercase “bitcoin” refers to the digital currency.
The Bitcoin system is not controlled by a single entity, like a central bank, and this has led the US Treasury to call bitcoin a decentralized currency.
Bitcoin wallet software, sometimes called a Bitcoin client software, allows a user to transact bitcoins. A wallet program generates and stores private keys, and communicates with peers on the Bitcoin network.
The first wallet program called Bitcoin-Qt was released in 2009 by Satoshi Nakamoto as open source code. It can be used as a desktop wallet for payments or as a server utility for merchants and other payment services. Bitcoin-Qt, also called “Satoshi client” is sometimes referred to as the reference client because it serves to define the Bitcoin protocol and acts as a standard for other implementations. As of version 0.9, Bitcoin-Qt has been renamed “Bitcoin Core” to more accurately describe its role in the network.When making a purchase with a mobile device, QR codes are used ubiquitously to simplify transactions. Several server software implementations of the Bitcoin protocol exist. So-called “full” nodes on the network validate transactions and blocks they receive, and relay them to connected peers.