What is a blockchain?
A blockchain is a database that is agreed upon by many machines instead of trusted to one operator. Every participant can check every entry against public rules.
In this lesson
- Define a blockchain without jargon
- Explain decentralisation in one sentence
- Name three properties that make a ledger trustworthy
A ledger everyone can check
A blockchain is a shared, append-only record of transactions, stored identically on thousands of computers. Instead of trusting a bank's private database, participants trust rules everyone can verify: valid signatures, no double-spending, and a deterministic process for picking the next block.
| Property | What it buys you |
|---|---|
| Decentralised | No single operator can freeze, censor or quietly edit your balance. |
| Append-only | New entries are added; old ones are only changed by rewriting history in public. |
| Transparent | Anyone can audit the full history — balances are computed, not revealed. |
| Pseudonymous | Addresses are public, identities are not — until on-chain forensics links them. |
Example · A shared spreadsheet with referees
Imagine a Google Sheet that 10,000 people hold a copy of. Every edit must be signed by the sender and validated by everyone before it is appended. If one copy is tampered with, the other 9,999 reject it. That is the essence — the "referees" are the consensus rules, not employees.
Not a magic trust machine
The chain guarantees the history is honest. It cannot guarantee that the person who sent you coins owns them, that a token has value, or that a smart contract does what its website claims.
Add blocks yourself
Mine a few blocks in the lab, then try to edit an old one and watch the chain disagree with you.
Is a blockchain just a database?
Technically yes — a replicated log with consensus rules. The innovation is reaching agreement without a central administrator.
Who runs it?
Anyone who wants to: volunteers, companies, miners or validators, depending on the network. New nodes can join and independently re-verify everything from the genesis block.
Does every blockchain work the same?
No. Parameters differ (block size, consensus, privacy), but the hash-linked, rule-verified structure is shared by Bitcoin, Ethereum and most others.
Don't trust — verify.