Crypto · Beginner · 4 min

What is a blockchain?

You can explain what a blockchain records, why many computers keep copies of it, and how new entries are agreed without a central authority.

One book, many copies

A bank keeps its own records of who owns what. If you want to know your balance, you trust the bank's books. A blockchain works differently: it is a record that thousands of independent computers keep together, with no single owner.

That idea is the foundation of every crypto asset in this rubric. This lesson explains it without hype: what a blockchain records, how it grows, and what it does not do.

Linked blocks, shared rules

A blockchain is a shared record of transactions, organized in blocks. Bitcoin's founding paper from 2008 described the core design:

Transactions are collected and grouped into a block.
Each block contains a reference to the previous block, a kind of digital fingerprint. Together, the blocks form a chain.
Many computers, called nodes, keep full copies of the chain and check every new block against the same rules.

Who decides which block comes next? There is no central authority. Instead, a consensus mechanism sets the rules by which participants agree:

Proof of work: computers compete to solve a hard computing puzzle, and the winner may add the next block. Bitcoin uses this. It consumes a lot of electricity.
Proof of stake: participants lock up coins as a deposit and are chosen to propose and confirm blocks. They can lose part of the deposit if they cheat. Ethereum switched to this in 2022.

Because each block is linked to the one before, changing an old entry would break all later links. And because many independent copies exist, the rest of the network would reject a changed version. To rewrite history, an attacker would need to control most of the network's computing power or stake.

What a blockchain does not do: it does not check whether a project is honest, whether a coin has any lasting value, or whether a price is fair. It only records transactions according to its rules.

Alice → Bob, recorded

Alice wants to send one coin to Bob on a fictional public blockchain.

She signs the transaction with her private key and broadcasts it to the network.
It waits with other transactions until it is packed into a new block.
The network agrees on the block by its consensus rules and adds it to the chain.

Every copy of the blockchain now shows that the coin moved from Alice to Bob. Anyone can see the transaction, though usually only as addresses, not names.

Common mistakes

"Blockchain means safe investment." The record can be secure while the price collapses.
"Transactions are anonymous." On public chains, they are visible to everyone, linked to addresses.
"Every blockchain is decentralized." Some are run by a small group or a single company.
"Mistakes can be reversed." A confirmed transaction usually cannot be undone.

The takeaway

A blockchain is a shared record of transactions, kept by many computers. New blocks are agreed by fixed rules, so nobody can quietly rewrite history.

Check yourself

Quick quiz

4 questions. Instant feedback, nothing is saved.

1Tap the step in which the network decides that a new block is valid.

Tap the chart.

A consensus mechanism, such as proof of work or proof of stake, decides which block is accepted.
2Why is it hard to change an old entry on a large public blockchain?

Changing one block would break the links to all later blocks, and the other copies would not accept it.
3Which two consensus mechanisms are the best known?

Bitcoin uses proof of work. Ethereum switched to proof of stake in 2022.
4Does a blockchain make the coins on it a safe investment?

A reliable record says nothing about what the asset is worth.

of 4 correct.