A blockchain is a distributed, append-only ledger of transactions grouped into blocks, where each block is cryptographically linked to the previous one. This structure makes the recorded history tamper-evident and verifiable by anyone without trusting a central authority.
What Is a Blockchain?
2 min read
The short version
Picture a shared Google Doc that everyone can read but nobody can edit past entries, only add new lines at the bottom. Each new line includes a fingerprint of the previous line, so changing anything old would break the chain of fingerprints and everyone would instantly notice.
How It Works
A blockchain consists of blocks connected by hash pointers. Each block header contains the hash of the previous block, a timestamp, and a Merkle root summarizing its transactions. Nodes in the network independently validate new blocks against consensus rules before appending them. Because every node holds a copy and validates independently, no single party can unilaterally alter history. The specific consensus mechanism (proof of work, proof of stake, etc.) determines how new blocks are proposed and agreed upon. Different blockchains make different design tradeoffs between decentralization, throughput, and finality speed.
How tampering is detected
Suppose an attacker changes one transaction in block 500,000 of Bitcoin. The Merkle root of that block changes, which changes the block header hash. But block 500,001 stores block 500,000's hash, so now block 500,001 is invalid too. The attacker must re-mine every block from 500,000 onward (currently 350,000+ blocks) while outpacing the honest network. At 600 EH/s honest hash rate, this would require more energy than a small country produces in a year, making it economically irrational.
What People Get Wrong
Blockchain and Bitcoin are the same thing
Bitcoin uses a blockchain, but blockchains are a broader data structure. Ethereum, Solana, and hundreds of other systems use blockchains with different designs and purposes.
Blockchains are always decentralized
The degree of decentralization varies enormously. Some blockchains have very few validators. The data structure alone doesn't guarantee decentralization, the network design does.
Blockchains are slow and wasteful by nature
Throughput and energy use depend on the consensus mechanism. Proof-of-stake chains finalize thousands of transactions per second with minimal energy.
Everything on a blockchain is encrypted
Most blockchains are transparent, transactions are publicly readable. The data is signed (authenticated) but not encrypted (hidden).
Keep Reading
Sources & Further Reading
- Ethereum.org: What is a Blockchain?
Ethereum Foundation introduction covering blockchain architecture
- Bitcoin Developer Guide
Technical documentation on blockchain structure and validation
Questions People Also Ask
- What is the difference between a public and private blockchain?
- Public blockchains (Bitcoin, Ethereum) are permissionless, anyone can participate. Private/permissioned blockchains restrict who can validate or read data, trading decentralization for speed and privacy.
- Can blockchains be deleted?
- By design, no. Blockchain data is append-only. Some chains support "pruning" old data from individual nodes, but the canonical history remains on archive nodes and is always recoverable.
- Do all blockchains use proof of work?
- No. Proof of work (Bitcoin), proof of stake (Ethereum, Solana), delegated PoS, proof of authority, and many other consensus mechanisms exist. Each has different tradeoffs.