What is a blockchain?

A blockchain is a shared record of transactions that many computers keep at the same time. Instead of one company holding the books, thousands of independent computers, called nodes, each hold a full copy.

Blocks, chained together

Transactions are grouped into blocks. Every block includes a short fingerprint (a hash) of the block before it. Change anything in an old block and its fingerprint changes, which breaks the link to every block after it. That is why past records are very hard to rewrite.

Agreeing without a boss

Nodes need a way to agree on which block comes next. This is called consensus:

• Proof of Work (used by Bitcoin): miners compete to solve a costly computing puzzle. The winner adds the next block and earns a reward.
• Proof of Stake (used by Ethereum since 2022): validators lock up coins as a deposit. They are chosen to propose blocks and lose part of the deposit if they cheat.

Public, but not named

Anyone can read most blockchains. You see addresses and amounts, not names. That makes blockchains pseudonymous, not anonymous: if an address is ever linked to you, its whole history is visible.

Key takeaways

• A blockchain is a shared ledger copied across many nodes.
• Each block links to the previous one, so history is hard to change.
• Consensus (Proof of Work or Proof of Stake) decides who adds the next block.