Picture a notebook that thousands of people keep an identical copy of. When someone adds a line, every copy updates, and everyone can check the maths for themselves. That is roughly what a blockchain does, except the notebook is software, the lines are transactions, and the checking is done by cryptography rather than by trust in a bookkeeper.
Entries are bundled into a <a href="/glossary/block/">block</a>. Each block carries a hash, a compact fingerprint, of the block that came before it. Alter one old transaction and its fingerprint changes, which breaks the fingerprint of every block that followed. Tampering is therefore easy to spot and very expensive to hide, which is where the word <em>immutable</em> comes from.
The second half of the design is agreement. Independent machines called nodes run the same rulebook and settle on one version of history through a <a href="/glossary/consensus/">consensus</a> mechanism. There is no admin button and no head office. The rules change only when enough participants voluntarily adopt new software, which is slow on purpose.
Two honest caveats are worth carrying with you. First, a blockchain guarantees that something was recorded, not that the record is true; nonsense written to a chain stays there as permanent nonsense. Second, most public chains are radically transparent, and anyone can read an address and its full history through a <a href="/glossary/block-explorer/">block explorer</a>. Decentralisation buys resistance to censorship, not privacy.
What Is Crypto and Blockchain? A Plain-English Start
Key takeaways
- The security comes from the linking: changing an old block means redoing every block that came after it, in public, faster than everyone else.
- Immutability protects the record itself, never the accuracy of what somebody chose to record.
- Treat a public address as a pseudonym rather than a secret, because its balance and history are readable by anyone who looks.
Blockchain — frequently asked questions
Is a blockchain the same thing as Bitcoin?
No. Bitcoin was the first widely used blockchain, but the word now covers many separate networks with different rules, speeds and costs. Bitcoin is one application of the idea, roughly the way email is one application of the internet. Some chains focus on payments, others on programmable contracts, and their designs are not interchangeable in practice.
Can information on a blockchain be deleted?
Not in any ordinary sense. Once a transaction sits under later blocks, erasing it would mean persuading the whole network to rebuild that stretch of history, which is deliberately impractical. Some networks prune old data for storage reasons and some store little on the chain itself, but you should assume anything you publish to a public chain is permanent.
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