In 30 seconds
- A blockchain is a shared record: thousands of computers keep the same copy and update it together.
- Transactions are grouped into blocks, and each block includes the fingerprint (hash) of the previous one: that's what chains them.
- Changing an old block breaks every fingerprint that comes after it, and the network rejects it.
- Bitcoin was the first (2009). Ethereum added programs that run on the chain (2015).
A record everyone has
In Chile, if you want to know who owns a property, you go to the Conservador de Bienes Raíces, the land registry: one institution keeps the record and we all trust it. A blockchain does the same job —keeping a record of who owns what— but without a central office: the record is copied across thousands of computers, every new entry is added to all the copies at once and nobody can tear out a page without the rest noticing.
Those computers are called nodes. On public blockchains, such as Bitcoin or Ethereum, anyone can run one: you don't need to ask for permission. That's why they are called permissionless networks and why nobody controls them alone.
Blocks, hashes and the chain
Transactions aren't recorded one at a time: they are grouped into blocks. Ethereum produces a block every 12 seconds; Bitcoin, one every 10 minutes on average.
The hash, a digital fingerprint
A hash is a function that turns any data —a word, a book, an entire block— into a fingerprint of fixed length. It has two key properties: the same input always produces the same fingerprint, and the slightest change to the input produces a completely different fingerprint. Bitcoin uses the SHA-256 function; Ethereum, Keccak-256.
How they are chained
Besides its transactions, each block stores the hash of the previous block. So block 1,042 “points” to 1,041, which points to 1,040, and so on all the way back to the very first.
Why it can't be erased
Say someone wants to erase an old payment. By changing that block, they change its hash. The next block stores the old hash, so the link breaks. To fix it they would have to redo that block and every block after it, and also convince the rest of the network that their version is the real one. Meanwhile, the network keeps adding blocks on top of the honest version.
More than “impossible to erase”, a blockchain is so expensive to rewrite that it isn't worth trying.
On Ethereum, a block is finalized within minutes. Reverting it afterwards would require the attacker to lose at least a third of all the ETH staked by validators: a figure that makes the attack economically absurd.
Who decides which block goes in? Consensus
Since there is no boss, the network needs a rule to agree on what the next block is. That rule is the consensus mechanism, and there are two big families:
| Mechanism | How it works | What a cheater risks | Example |
|---|---|---|---|
| Proof-of-work | Computers compete to solve a problem that requires spending energy; the winner proposes the block. | The electricity and equipment invested; they would need more computing power than the rest of the network. | Bitcoin |
| Proof-of-stake | Validators put money up as collateral (32 ETH each on Ethereum) and take turns proposing and confirming blocks. | Their collateral: if they act in bad faith, the network takes part of it. | Ethereum, since September 15, 2022 |
Ethereum switched from proof-of-work to proof-of-stake in September 2022, in an upgrade called The Merge, without stopping the network.
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Public, private and transparent
- Public: anyone can read it and anyone can take part. Bitcoin and Ethereum are public.
- Private or consortium: only authorized entities take part. It works between companies, but loses part of the point: you have to trust whoever runs it.
- Transparent is not the same as anonymous: everything you do with an address stays visible forever. Your name doesn't appear, but if someone links your address to you, they can see your entire history.
What it's good for (and what it isn't)
| Good for | Not (so) good for |
|---|---|
| Records that no single party should control | Storing heavy files: every byte costs money |
| Histories that anyone can verify | Data that must stay private or that the law requires to be erasable |
| Digital assets that move without intermediaries | Systems that already have a trusted, efficient authority |
| Rules that execute themselves (smart contracts) | Solving problems that are really about trust between people |
Where to go next
Bitcoin showed that a blockchain can move money. Ethereum asked what would happen if it could also run programs. That's the next block: Ethereum, smart contracts and gas. If a word is still bugging you, look it up in the glossary.
FAQ
Are blockchain and Bitcoin the same thing?
No. Bitcoin is a blockchain —the first one— and also the name of its currency. There are many other blockchains, such as Ethereum, which besides moving money runs programs.
Can a blockchain be hacked?
Rewriting a large blockchain is extremely expensive. What does get attacked often are the pieces around it: exchanges, bridges between networks, contracts with bugs and, above all, people tricked into signing something.
Who owns a public blockchain?
Nobody in particular. It is maintained by those who run nodes and validate blocks, under open-source rules that anyone can review.
Sources
- Satoshi Nakamoto · Bitcoin: A Peer-to-Peer Electronic Cash System (2008)
- ethereum.org · Blocks (accessed Sep 10, 2026)
- ethereum.org · Proof-of-stake (accessed Sep 10, 2026)
- ethereum.org · Ethereum history (The Merge) (accessed Sep 10, 2026)
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