Bitcoin: A Peer-to-Peer Electronic Cash System

Digital cash has an obvious problem: a digital file is trivially easy to copy, so what stops someone from spending the same digital coin twice? Historically this was solved by having a trusted central authority (a bank) keep the official ledger of who owns what. Nakamoto's proposal removes the need for that trusted third party entirely: every transaction is broadcast to a network of computers, bundled into 'blocks,' and each block is cryptographically chained to the one before it. Network participants ('miners') compete to solve a computationally expensive puzzle to add the next block, and the network simply agrees that whichever chain has the most accumulated computational work behind it is the true history.