What Is a Blockchain?

A blockchain is a ledger, a list of who sent what to whom, copied across thousands of computers instead of sitting in one company's database. Every so often the network bundles new transactions into a block, links it to the previous block, and every copy of the ledger updates. Once a block is buried under enough newer blocks, changing it would mean redoing the work of the entire network from that point forward, which is somewhere between economically absurd and physically impossible. That's the whole trick: a record that everyone can read and nobody can quietly edit.

If you've ever waited on a bank transfer clearing "in 3 to 5 business days", you already understand the problem blockchains were built to solve. Traditional ledgers are private, reconciled slowly between institutions, and changeable by whoever controls them. A blockchain is public, reconciled continuously by the network itself, and changeable by nobody in particular. Whether that trade is a good deal depends entirely on what you're using it for, and we'd encourage healthy scepticism toward anyone who says a blockchain improves everything it touches.

Blocks, chains, and why the name is literal

Each block contains a batch of transactions plus a cryptographic fingerprint (a hash) of the block before it. Change a single character in an old block and its fingerprint changes, which breaks the link to every block after it. That's what makes the history tamper-evident. The network's job is agreeing on which new block gets added next, and the two dominant ways of doing that are proof of work, the mining system Bitcoin uses, and proof of stake, which Ethereum switched to in September 2022.

A useful mental model: the blockchain is the courtroom record, not the courtroom. It can prove that a transaction happened, in a certain order, signed by a certain key. It cannot prove the transaction was wise, that the counterparty was honest, or that the token you bought has any value. Plenty of scams have run entirely on-chain, in full public view, recorded immutably for posterity. Immutable and trustworthy are not the same word.

What actually lives on a blockchain

Less than you'd think. On Bitcoin, essentially just balances and transfers. On chains like Ethereum, there are also smart contracts, programs whose code and state live on-chain, which is what makes DeFi possible. What almost never lives on-chain is the stuff people assume does: images attached to NFTs, the reserves backing a stablecoin, the identity of whoever controls a wallet. When you hear "it's on the blockchain" as a guarantee, ask precisely what "it" is.

Public and permissionless, with consequences

Anyone can use a public blockchain, and nobody can un-send a transaction. Both halves of that sentence carry weight. The openness is radical in the plain sense: a farmer with a phone has the same access as a hedge fund. The finality is just as radical in the other direction. Send funds to a wrong address and there is no support line, no chargeback, no undo. We cover what that means for you practically in self-custody and in our guide to storing a seed phrase. Read both before real money gets involved.

One more thing to internalise early: each blockchain is its own network. Bitcoin doesn't know Ethereum exists. Moving value between chains requires bridges, which have historically hosted some of the largest hacks in the industry. Different chains also make sharply different trade-offs between speed, cost, and decentralization, which is the reason layer 2 networks exist at all.

Where this leaves you

You don't need to understand elliptic curves to use crypto safely, the same way you don't need to understand TCP/IP to send an email. What you do need is the operating picture: a public, append-only ledger; keys instead of accounts; finality instead of chargebacks; and a strict boundary between what the chain guarantees (the record) and what it doesn't (everything else). Every price, market cap, and indicator on our coin pages ultimately derives from activity settling on these ledgers. Every risk warning we attach comes from the parts the ledger can't protect you from.

Frequently asked questions

Who controls a blockchain?

Nobody in particular, and that's the point. The rules are enforced by every participant running the same software, and changing those rules requires broad agreement across the network. Individual companies can control apps built on a blockchain, but the ledger itself has no owner with an edit button.

Can a blockchain be hacked?

The major public chains have never had their core ledgers rewritten. What gets hacked is everything around them: exchanges, bridges, smart contracts with bugs, and people tricked into signing things. The record is hard to break. The stuff people build on top of it breaks constantly.

Is blockchain the same thing as Bitcoin?

No. Bitcoin was the first blockchain and remains the largest cryptocurrency, but thousands of separate blockchains now exist, each its own network with its own rules. Bitcoin is one application of the idea; Ethereum, Solana, and the rest are others.