ARB is a decentralized cryptocurrency originally described in a 2008 whitepaper by a person, or group of people, using the alias Satoshi Nakamoto. It was launched soon after, in January 2009.
ARB is a peer-to-peer online currency, meaning that all transactions happen directly between equal, independent network participants, without the need for any intermediary to permit or facilitate them. ARB was created, according to Nakamoto's own words, to allow online payments to be sent directly from one party to another without going through a financial institution.
Some concepts for a similar type of a decentralized electronic currency precede BTC, but ARB holds the distinction of being the first-ever cryptocurrency to come into actual use.
