What Is a Blockchain?

What a blockchain is

A blockchain is a shared digital ledger that records transactions in order and is designed so that records can only be added, never secretly rewritten. If you've read our guide on what cryptocurrency is, you've already met the idea: instead of one bank keeping a private record of who owns what, a blockchain keeps a public record that many computers agree on.

The name describes the structure. Transactions are bundled into "blocks," and each new block is attached to the one before it, forming a chain that stretches all the way back to the very first block. Because the chain only ever grows from the end, the history accumulates like rings in a tree — you can add to it, but you can't slip something into the middle without everyone noticing.

What's inside a block

A block is just a container for data, plus a few pieces of bookkeeping that make the chain work. While details differ between networks, most blocks hold four key things:

• Transactions — the batch of recent activity being recorded, such as "address A sent 0.5 coins to address B."
• A timestamp — roughly when the block was created, which fixes its place in the timeline.
• Its own hash — a unique fingerprint calculated from everything inside the block.
• The previous block's hash — a copy of the fingerprint of the block that came before it.

That last item is the clever part. By storing the previous block's fingerprint, every block points backward to its parent, which is exactly what stitches the separate blocks into a single, ordered chain.

How blocks link via hashing

To understand why a blockchain is so hard to tamper with, you need one concept: the hash.

Each block's hash is calculated from its contents, including the previous block's hash. So the blocks are chained by fingerprints: block 100 contains block 99's hash, block 99 contains block 98's hash, and so on.

Now imagine an attacker tries to rewrite an old transaction in block 50. The moment they change that data, block 50's hash changes. But block 51 still stores the old hash of block 50, so the link no longer matches and the tampering is obvious. To cover their tracks, the attacker would have to recalculate block 50's hash, then block 51's, then block 52's — every single block all the way to the tip of the chain — and do it faster than the rest of the network is adding new blocks. On a healthy network, that's practically impossible. This is why people say a blockchain is "immutable": not because editing is forbidden, but because editing is absurdly expensive.

Decentralization & nodes

A blockchain isn't kept on one company's server. It lives across thousands of independent computers called nodes, scattered around the world. Each node stores its own full copy of the ledger and checks new transactions against the rules — for example, confirming that you actually own the coins you're trying to spend.

This is what "decentralized" really means: there is no single point of control and no single point of failure. If one node goes offline or behaves dishonestly, the rest carry on. A change is only accepted if it follows the shared rules that everyone's software enforces, so no individual operator can rewrite balances or freeze your funds. It also means the network is hard to switch off — there's no central plug to pull.

Consensus: proof of work vs proof of stake

If thousands of nodes all hold a copy of the ledger, they need a way to agree on which new block to add next — without a boss to settle disputes. That agreement process is called consensus. Two approaches dominate:

Proof of work

Computers compete to solve a hard mathematical puzzle. The first to solve it earns the right to add the next block and a reward. Solving the puzzle takes real electricity and hardware, which makes cheating costly: to rewrite history you'd have to out-compute the entire honest network. Bitcoin is the best-known example.

Proof of stake

Instead of burning energy, participants lock up — or "stake" — their own coins as collateral for the chance to add blocks. If they validate honestly, they earn rewards; if they cheat, they can lose their stake. This achieves security through financial skin in the game rather than raw computation. Ethereum uses this model; see Bitcoin vs Ethereum for how the two compare.

What blockchains are good (and bad) at

Blockchains solve a specific problem well, but they aren't the right tool for everything. It helps to be clear-eyed about both sides.

Strengths:

• Tamper-evidence — past records can't be quietly altered without detection.
• No central gatekeeper — no single authority can block or reverse a valid transaction.
• Transparency — on public chains, anyone can inspect and verify the full history.
• Resilience — with copies spread across many nodes, there's no single point of failure.

Limitations:

• Speed and scale — agreeing across many nodes is slower than a single private database.
• Cost — securing the network (through energy or staked capital) isn't free.
• Finality of mistakes — sending to the wrong address is usually irreversible.
• Not always necessary — if you trust a central party, a normal database is simpler and cheaper.

Public vs private blockchains

Not every blockchain is open to the world. Public blockchains, like those behind major cryptocurrencies, let anyone read the ledger, submit transactions and run a node. They maximize openness and censorship-resistance, and they're where you'll hold assets in a self-custody wallet.

Private (or permissioned) blockchains restrict who can participate — typically a known group of companies or institutions. They trade some openness for speed and control, and are often used inside businesses for shared record-keeping. The core mechanics of blocks and hashing are the same; what differs is who's allowed in.

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