A blockchain is a shared record book that thousands of strangers can keep in sync without trusting each other. Bitcoin's is the original, and still the most secure.
Blocks
Every ten minutes or so, a miner gathers waiting transactions into a block. A block is just a list: "this address paid that address this much", plus a little header.
The chain
Each header contains a hash — a digital fingerprint — of the previous block. Change one character in an old block and its fingerprint changes, which breaks the link to every block after it. That's why history is so hard to rewrite.
Proof of work
To add a block, miners must find a number that makes the header's hash start with enough zeros. The only way is guessing, trillions of times per second, which costs electricity. Whoever finds it first earns newly issued bitcoin plus the transaction fees.
To rewrite the past, an attacker would need to redo all of that work faster than the rest of the world combined. Today that would cost billions of dollars in hardware and energy.
Nodes keep miners honest
Anyone can run a node — software that downloads every block and checks every rule. If a miner breaks the rules (say, creating extra coins), nodes simply reject the block. Miners provide security; nodes decide what the rules are.
A word on "blockchain, not bitcoin"
In 2017 every company wanted a blockchain for land records, supply chains and voting. Most of those projects quietly ended. A blockchain is slow and expensive by design; that trade-off only makes sense when there's no one you can trust to keep the database. For money without a central issuer, that's exactly the point. For most other things, a normal database is better.
Originally published November 2017 — rewritten September 2026.