Blockchain

A blockchain is a record of transactions copied across thousands of computers at once. No single company runs it. No one machine holds the only copy. That spread-out design is the whole point.

The name describes the structure. Transactions are bundled into blocks, and each block carries a cryptographic fingerprint of the block before it. Change one old record and every fingerprint after it breaks, so tampering shows up instantly. The blocks form a chain running back to the very first one.

New blocks get added on a fixed rhythm. On Bitcoin a fresh block lands roughly every 10 minutes. The computers maintaining the network check each transaction against the shared history before it is accepted, which stops anyone spending the same coin twice. This validation work is what people mean by mining on networks that use proof-of-work.

Most public blockchains are open to read. You can look up any past transaction without an account or permission. What you cannot do is fake one, because the rest of the network would reject it.

Not every blockchain does the same job. Bitcoin's was built to move money. Ethereum added programmable contracts that run code, not just payments. Each one keeps its own separate ledger, with its own rules and its own native asset.

The trade-off is speed and cost. Updating thousands of copies takes time, and when demand spikes, fees climb. A blockchain is slower than a bank database by design, and it gives you something a bank cannot: a shared record no single party can quietly rewrite.