What is blockchain? A simple explanation

A ledger is just a list of who owns what. Traditionally one organisation keeps it — a bank, a land registry, a company database — and everyone else has to trust that organisation to keep it correct.

A blockchain replaces the single keeper with a network. Every participating computer (a node) holds a full copy of the ledger and checks the same rules. When new transactions arrive, the network agrees on them together, so no single party can rewrite the record on its own.

Transactions are grouped into batches called blocks. Each block carries a timestamp, its list of transactions and a hash of the block before it. A hash is a fixed-length fingerprint computed from data: change a single character of the input and the fingerprint changes completely.

Because block N+1 contains the hash of block N, the blocks are locked together in order. That ordering is what turns a pile of records into a history.

The data isn't locked away; it is that changing it is detectable and prohibitively expensive. Honest nodes reject a rewritten history because its hashes no longer line up, and rewriting it convincingly would mean out-working the whole network. The longer a block has been buried, the safer it is — which is why payments wait for confirmations.

Cryptocurrency payments run on public chains: anyone can verify a transaction independently on a block explorer, without asking the merchant or the gateway.

Some blockchains, Ethereum first, can also run small programs called smart contracts. The code and its state live on the chain, so everyone can inspect it and nobody can alter it afterwards. That is how tokens such as USDT and USDC exist: a smart contract keeps the table of who holds how many, and enforces the rules for moving them.

• Payments and transfers — sending value across borders without intermediaries.
• Tokens — stablecoins and other assets issued on a shared network.
• Provenance — recording where goods came from, with entries that are hard to alter after the fact.
• Open applications — lending, trading and games whose rules are public code.

It is a poor fit where you simply need a fast private database. A blockchain trades speed and simplicity for shared trust, so it earns its place only when several parties who don't trust each other need one record.