Blockchain

A blockchain stores transaction records identically across many computers. Records are grouped into blocks linked in a chain, which is where the name comes from.

The key property is that it is hard to alter. Changing one block requires recomputing every block after it and winning agreement from a majority of the network, which makes forgery effectively impossible.

That allows trust without an intermediary — transferring money without a bank, proving ownership without a registry. Cryptocurrency was the first application of the idea.

It is used outside finance as well: tracing which farm food came from, verifying that a luxury good is genuine, or running smart contracts that execute agreements automatically.

The limits are equally clear. Because every participant validates the same records, it is slow and costly. Where banking systems handle tens of thousands of transactions per second, early blockchains managed a handful.