What is a blockchain, in simple terms?

A ledger that everyone keeps

Think of the ledger as a spreadsheet. It starts with a first share-out of coins, then records every transaction after that.² The BIS describes a distributed ledger as one where each user stores an up-to-date copy.² Each user can then check their own copy to see whether a payment really happened.²

Blocks, linked in a chain

NIST, the US standards body, describes blockchains as shared digital ledgers of signed transactions, grouped into blocks.¹ Each block is linked to the one before it, so any tampering shows.¹ As new blocks are added, older blocks get harder to change.¹

The link is made with a hash. A hash function turns any data into a short code. Change even one tiny bit of the data, and the code comes out completely different.¹

Each block carries the hash of the block before it.^1,3 The very first block, called the genesis block, has no block before it.¹

Who adds the blocks

Each computer in a blockchain network is called a node. A full node stores the whole chain and checks that transactions are valid.¹ In Bitcoin's design, new transactions are sent to all nodes.³ Each node collects transactions into a block. Nodes accept a block only if every transaction in it is valid and not already spent.³

Nodes always treat the longest chain as the correct one.³ To change an old block, an attacker would have to redo the work for that block and every block after it. Then they would have to catch up with, and overtake, the honest nodes.³

The double-spending problem

Digital money is easy to copy, so someone could try to spend the same money twice.² Before cryptocurrencies, the only fix was a central party that kept a record of all transactions and checked each one.²

The Bitcoin paper's answer was to announce every transaction publicly. Everyone then agrees on one history of the order in which payments arrived.³ If the same coins are spent twice, the earliest payment is the one that counts.³

That paper was published in 2008 under the name Satoshi Nakamoto, and the Bitcoin network started in 2009.¹