What is blockchain?
Written by an ex-institutional trader. A plain-English explanation of blockchain: what it is, how blocks are chained with cryptography (shown with a diagram), why that makes it tamper-evident, and where it is actually used.
Direct answer
A blockchain is a shared digital ledger that records transactions in batches called blocks, each cryptographically linked to the one before it, and maintained across a decentralised network of computers. Because every block contains a fingerprint (a hash) of the previous block, changing any past record would break the chain and be instantly detectable. That is what makes a blockchain tamper-evident and removes the need for a single trusted middleman like a bank.
Blockchain is the technology underneath Bitcoin and other cryptocurrencies, but the idea is broader: any record that benefits from being shared, permanent and hard to falsify can use one. Its strengths are transparency, security through decentralisation, and resistance to tampering. Its trade-offs are speed and scale, since thousands of computers agreeing on every update is slower than a single database. Understanding the blockchain is the foundation for understanding crypto.
What a blockchain is
A blockchain is a shared digital ledger that records transactions in batches called blocks, each cryptographically linked to the one before it, and maintained across a decentralised network of computers. Instead of one organisation holding the master copy, every participant holds the same copy, and they agree on updates together.
It is the technology underneath Bitcoin and other cryptocurrencies, and it is what lets digital money work without a bank keeping the central record. But the idea is more general: any record that benefits from being shared, permanent and hard to falsify can use a blockchain.
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How it works
Transactions are grouped into a block. Each block is given a hash, a unique fingerprint calculated from its contents, and it also stores the previous block's hash. That shared reference is the link that chains the blocks together, in order, all the way back to the first.
Every computer on the network holds a copy of this chain, and they agree on its current state through a process called consensus. New blocks are added roughly on a schedule (about every 10 minutes for Bitcoin), and once a block has others built on top of it, it is effectively permanent.
Why it is tamper-evident
The chaining is the clever part. Because each block's hash depends on its exact contents, changing any past transaction would change that block's hash. But the next block stored the old hash as its "previous" reference, so the link would no longer match, and every block after it would break too.
To rewrite history, an attacker would have to recompute every following block and do it faster than the rest of the network combined, across thousands of independent copies, all at once. For a large, well-distributed blockchain that is practically impossible. This is why a blockchain is described as tamper-evident: not that it is literally impossible to alter, but that any alteration is immediately obvious and rejected by the network. It removes the need to trust a single central record-keeper.
What it is used for
The original and still-dominant use is cryptocurrency, where the blockchain records who owns what without a bank. But the same properties, shared, permanent, tamper-evident, apply elsewhere:
- Smart contracts and DeFi. Programmable blockchains like Ethereum run code that executes automatically, powering decentralised finance and NFTs.
- Supply chains. Tracking goods from origin to shelf, with a record that cannot be quietly altered.
- Identity and registries. Digital identity, land titles and document verification.
A useful note of scepticism: many things pitched as needing a blockchain would work just as well, and faster, on an ordinary database. Blockchain earns its keep specifically when no single party should be trusted to control the record.
The trade-offs
Blockchain is not free magic. Its strengths come with real costs:
- Speed and scale. Having thousands of computers agree on every update is slower than one database. This is the main engineering challenge crypto networks work to solve.
- Energy (for some). Proof-of-work blockchains like Bitcoin use significant energy; newer designs like proof-of-stake use far less.
- Irreversibility. The permanence that makes it secure also means mistakes cannot be undone.
- The apps on top. The blockchain itself may be secure while the exchanges and wallets built on it are not, which is where most real-world losses happen.
Understanding the blockchain is the foundation for understanding crypto. From here, what is cryptocurrency covers the money built on it, and what is a crypto wallet covers how you hold it safely. When you are ready to buy, use a reputable AUSTRAC-registered exchange.
Popular Australian crypto exchanges
All three are AUSTRAC-registered Australian exchanges. Crypto is volatile; only invest what you can afford to lose.
Diagram is illustrative. This is general information, not financial advice. Last reviewed: 2026-06-02.
Frequently asked questions
What is blockchain in simple terms?
A blockchain is a shared digital record book that many computers keep a copy of at once. Transactions are grouped into blocks, and each block is chained to the previous one using a cryptographic fingerprint called a hash. Because the copies are spread across a decentralised network and the blocks are linked, no single party controls it and past records cannot be quietly changed. It is the technology that lets cryptocurrencies work without a bank keeping the master ledger.
How does a blockchain work?
Transactions are collected into a block. The network verifies them, and the block is given a hash, a unique fingerprint calculated from its contents. Each new block also stores the previous block's hash, linking them into a chain. Every computer on the network holds a copy of the chain and they agree on its state through a consensus process. Once a block is added and built upon, altering it would change its hash and break every link after it, which the whole network would immediately reject.
What is a hash in blockchain?
A hash is a fixed-length fingerprint generated from data by a cryptographic function. Any change to the input, even a single character, produces a completely different hash. In a blockchain, each block has its own hash and also records the previous block's hash, which is what links the blocks together. Hashes are what make a blockchain tamper-evident: if someone altered an old transaction, that block's hash would change, breaking its link to every following block and exposing the tampering instantly.
Is blockchain the same as Bitcoin?
No. Bitcoin is a cryptocurrency; blockchain is the underlying technology it runs on. Bitcoin was the first application of a blockchain, but the two are not the same thing. Many other cryptocurrencies use their own blockchains, and blockchains are also used for non-currency purposes such as supply-chain tracking, digital identity and record-keeping. Think of blockchain as the engine and Bitcoin as one car built on it.
Why is blockchain considered secure?
Its security comes from two things: decentralisation and cryptographic linking. Because the ledger is copied across many independent computers, there is no single server to hack or shut down, and a majority of the network would have to be compromised at once to falsify records. Because each block is linked to the previous one by its hash, changing any past record breaks the chain in a way the network detects and rejects. Together these make a well-run blockchain extremely resistant to tampering, though the apps and exchanges built on top can still be vulnerable.
What is blockchain used for besides cryptocurrency?
Beyond crypto, blockchains are used or trialled wherever a shared, tamper-evident record is valuable: supply-chain tracking (proving where goods came from), digital identity, land and property registries, voting systems, and verifying the authenticity of documents or goods. Smart-contract blockchains like Ethereum also power decentralised finance and other applications. That said, many proposed uses do not actually need a blockchain over a normal database, so it is worth being sceptical of blockchain for its own sake.
