Source: Bitcoin Wikipedia Page

Bitcoin is a worldwide cryptocurrency and digital payment system[13]:3 invented by an unknown programmer, or a group of programmers, under the name Satoshi Nakamoto.[14] It was released as open-source software in 2009.[15]

The system is peer-to-peer, and transactions take place between users directly, without an intermediary.[13]:4These transactions are verified by network nodes and recorded in a public distributed ledger called a blockchain. Since the system works without a central repository or single administrator, bitcoin is called the first decentralized digital currency.[13]:1[16]

Besides being created as a reward for mining, bitcoin can be exchanged for other currencies,[17] products, and services in legal or black markets.[18][19]

As of February 2015, over 100,000 merchants and vendors accepted bitcoin as payment.[20] According to research produced by Cambridge University in 2017, there are 2.9 to 5.8 million unique users using a cryptocurrency wallet, most of them using bitcoin.[21]