A blockchain is a shared database that many computers keep in sync, where new records are added in batches called «blocks» and old records cannot be quietly changed. Each block contains a cryptographic fingerprint of the one before it, so the blocks form a chain. If anyone tampers with an old block, the fingerprints stop matching and the rest of the network rejects the change.
Blockchains are the technology behind cryptocurrency, but the idea is simple enough to understand in ten minutes. Let’s walk through it.
Key takeaways
- A blockchain is an append-only ledger copied across many independent computers (nodes).
- Blocks are linked by hashes, which makes rewriting history extremely difficult.
- A consensus mechanism — like proof of work or proof of stake — decides who adds the next block.
- Public blockchains are open to anyone; private ones restrict who can participate.
A simple analogy
Imagine a notebook shared by a group of friends to track who owes whom. Instead of one person keeping the notebook, everyone has an identical copy. When someone pays, they announce it to the group, everyone checks it, and everyone writes the same line in their copy. Once a page is full, it gets sealed with a unique stamp that depends on everything written on it and on the stamp of the previous page.
If someone later tried to change an old page in their copy, its stamp would no longer match — and neither would the stamps on every page after it. Everyone else would immediately see that copy is wrong. That’s essentially a blockchain.
The building blocks of a blockchain
1. Transactions
A transaction is a signed message such as «address A sends 0.1 BTC to address B». It’s signed with A’s private key, so anyone can verify that A authorized it without learning the key itself.
2. Blocks
A block is a bundle of transactions plus a header containing a timestamp, the hash of the previous block and other data. Bitcoin produces a block roughly every 10 minutes; Ethereum produces one every 12 seconds.
3. Hashes
A hash function turns any data into a fixed-length fingerprint. Change a single character of the input and the output changes completely. Because each block includes the previous block’s hash, the chain is tamper-evident.
4. Nodes
Nodes are computers running the blockchain’s software. Full nodes store a copy of the chain and independently verify every block against the rules. Anyone can run one, which is what keeps the system honest.
5. Consensus
Thousands of nodes need to agree on a single version of history. A consensus mechanism decides who gets to propose the next block and makes cheating expensive. The two big ones are compared in detail in proof of work vs proof of stake.
How a transaction gets onto the blockchain
- You create and sign a transaction in your wallet.
- Your wallet broadcasts it to the network, where it waits in a pool of pending transactions (the «mempool»).
- A miner or validator picks transactions — usually those paying higher fees first — and builds a block.
- The block is shared with the network. Nodes check every transaction and the block itself against the rules.
- If valid, nodes add the block to their copy of the chain. Your transaction now has one confirmation.
- Each additional block built on top adds another confirmation, making reversal even less likely.
Why blockchains are hard to tamper with
To rewrite a past transaction, an attacker would need to redo that block and every block after it, faster than the honest network keeps adding new ones. On Bitcoin, that means out-competing the combined computing power of miners worldwide; on Ethereum, it means controlling a huge share of all staked ETH. The cost makes such attacks impractical on large networks — though smaller blockchains have suffered so-called «51% attacks».
Public vs private blockchains
| Public (permissionless) | Private (permissioned) | |
|---|---|---|
| Who can join | Anyone | Approved participants only |
| Who validates | Open set of miners/validators | Known organizations |
| Transparency | Fully public ledger | Visible to members |
| Examples | Bitcoin, Ethereum, Solana | Enterprise and consortium chains |
What blockchains are used for
- Digital money — Bitcoin and other cryptocurrencies.
- Smart contracts — self-executing programs on Ethereum and similar platforms.
- Stablecoins and payments — dollar-pegged tokens that settle globally in minutes.
- Decentralized finance — lending, trading and saving without a bank (what is DeFi?).
- Tokenized assets and digital ownership — from NFTs to tokenized bonds.
Limitations of blockchains
- Scalability: Base layers process relatively few transactions per second, which is why Layer 2 networks exist.
- Finality and mistakes: There’s no «undo» button. A payment to a wrong address is usually lost.
- Privacy: Public ledgers expose every transaction to anyone who looks.
- Garbage in, garbage out: A blockchain guarantees the record wasn’t altered, not that the original information was true.
Frequently asked questions
Is blockchain the same as Bitcoin?
No. Bitcoin is one cryptocurrency that runs on its own blockchain. «Blockchain» is the general technology, used by thousands of networks.
Who owns a blockchain?
Nobody owns a public blockchain like Bitcoin or Ethereum. The software is open source and the network is run by independent node operators, miners or validators around the world.
Can a blockchain be deleted?
As long as at least some nodes keep a copy, the chain survives. Large public blockchains are replicated on tens of thousands of machines worldwide.
Next: see how the first blockchain works in practice in What is Bitcoin? Unfamiliar term? Check our crypto glossary.
