Bitcoin is a cryptocurrency and a digital payment system invented by an unknown programmer, or a group of programmers, under the name Satoshi Nakamoto. It was released as open-source software in 2009.
The system is peer-to-peer, and transactions take place between users directly, without an intermediary. These transactions are verified by network nodes and recorded in a public distributed ledger called the blockchain. Since the system works without a central repository or single administrator, bitcoin is called the first decentralized digital currency.
Besides being created as a reward for mining, bitcoin can be exchanged for other currencies, products, and services in legal or black markets. Source: Wikipedia