Crypto 101 · Crypto basics
What Is a Blockchain? A Plain-English Guide
Abstract
A blockchain is a record of transactions that many independent computers keep and check together, so no single company has to be trusted with it. We explain it first with an everyday picture, then show how blocks are linked by fingerprints called hashes and why rewriting history is so expensive on a proof-of-work network like Bitcoin or Litecoin.
Keywords: blockchain, block, hash, proof of work, distributed ledger
1. In plain English
Imagine a shared notebook that thousands of people each keep an identical copy of. Every time someone pays someone else, the payment is written on a new page. Before a page is added, everyone checks that the payment is valid, for example that the sender really has the money. Once the page is added, everyone updates their copy.
Nobody owns the notebook, and a bank does not sit in the middle. If one person tries to sneak a fake payment in, the thousands of other copies disagree and the fake page is ignored. That shared notebook is a blockchain. The pages are blocks, and the payments written on them are transactions.
2. Why the past is hard to change
Each page carries a short fingerprint of the page before it. A fingerprint, called a hash, is a fixed-length code computed from the page's contents: change even one letter and the fingerprint changes completely. Because every page contains the previous page's fingerprint, altering an old page would change its fingerprint and break every page after it. To hide the change, an attacker would have to redo all of that work faster than everyone else is adding new pages.
3. Go deeper: how the links work
A block header holds the hash of the previous header, a nonce and a Merkle root that summarises the block's transactions [1]. Bitcoin's design makes a header valid only if its double SHA-256 hash is below a target, and finding such a header is deliberately expensive while checking it is cheap [2, 3]. That expense is called proof of work, and it is what makes rewriting history costly: the honest network keeps adding blocks, and an attacker must out-work all of it to replace them.
Not every blockchain uses proof of work. Some choose block producers by how much of the network's own coin they have locked up, which is called proof of stake. The trade-offs are covered in proof of work versus proof of stake.
4. What this means for you
A transaction on a blockchain is public and, once it has enough later blocks on top of it, practically permanent. That is why sending crypto to a wrong address usually cannot be undone, and why you should read how transactions and confirmations work before sending a large amount.
References
- Bitcoin Project (2020). Block Chain, Bitcoin Developer Guide. Explains blocks, the block header, proof of work and how nodes choose the best chain. https://developer.bitcoin.org/devguide/block_chain.html
- Bitcoin Wiki contributors (2010). Block hashing algorithm. Bitcoin Wiki: the block header is hashed with SHA-256 applied twice and compared with the target. https://en.bitcoin.it/wiki/Block_hashing_algorithm
- Nakamoto, S. (2008). Bitcoin: A Peer-to-Peer Electronic Cash System. Whitepaper. https://bitcoin.org/bitcoin.pdf
Common questions
Is a blockchain the same as Bitcoin?
No. Bitcoin is a currency that runs on a blockchain. Many other blockchains exist, each with its own coin, rules and community.
Can a blockchain be changed?
Adding new blocks is normal. Changing old ones is extremely expensive on a proof-of-work chain because every later block would have to be redone.
General education, not financial, tax or legal advice. Figures are schematic. Protocol parameters are cited to primary sources and can change; verify against the linked source before relying on them.