A peer-to-peer electronic crypto-currency cash system
Bitcoin is a free open source peer-to-peer electronic cash system that is
completely decentralized, without the need for a central server or trusted
parties. Users hold the crypto keys to their own money and transact directly
with each other, with the help of a P2P network to check for double-spending.
See the documentation at the bitcoin wiki:
https://en.bitcoin.it/wiki/Main_Page
for help and more information.
This is the standard-client bitcoin-core.