Satoshi Nakamoto, the name used by the presumed pseudonymous person or persons who developed bitcoin describe it as: A Peer-to-Peer Electronic Cash System.
In short Bitcoin is a key.
Today Bicoin is a decentralized global internet digital currency not backed by any government or central bank that can be sent from user to user on the peer-to-peer bitcoin network without the need for intermediaries.
Bitcoin can be exchanged for traditional currencies and can act as an alternative to national fiat money and traditional commodities like gold.
More info can be fond on Wikipedia