A blockchain is how networks like Bitcoin or Ethereum keep a transaction history that no one can quietly rewrite.
A chain of blocks
New transactions aren’t written into the ledger one at a time. They’re grouped into batches called blocks, and each block, once confirmed, gets added to the end of the existing chain. Every block also contains a digital fingerprint of the block before it, produced by a calculation run over that block’s contents. Change a single character in an old block and its fingerprint changes completely, breaking the link to the block after it. That’s where the name comes from: the chain stays valid only as long as every link matches the next one exactly.
Who keeps the ledger
There’s no central computer holding the chain. Thousands of independent computers, called nodes, each keep a full copy and update it as new blocks appear. The nodes that propose new blocks, called miners on Bitcoin or validators on other networks, earn a fee for doing so; we cover that mechanism in the fees guide. If you want an address’s balance or a transaction’s history, you can ask any node: they all give the same answer, because they all start from the same chain.
Why it’s hard to fake
To change an old transaction, you’d have to rewrite not just the block that contains it but the fingerprint of every block after it, then convince most of the network’s nodes to accept your version instead of the real one. On large networks like Bitcoin, the computing cost of doing that outweighs any possible gain. It isn’t an absolute mathematical guarantee. It’s a practical cost high enough that, so far, no one has paid it successfully.
Public, but not anonymous
Anyone can download a node and read the chain from the very first block onward: the amounts transferred, the addresses involved, the timing of each transaction. What doesn’t show up is the real name behind an address. That’s why a blockchain is sometimes described as “pseudonymous” rather than “anonymous”: the address is public, but the identity behind it isn’t.
Each coin keeps its own
Bitcoin, Ethereum, and most well-known crypto-assets each run their own blockchain, with their own rules for confirming blocks. They don’t share one ledger - each network has its own chain, separate from the others, even though the general mechanism (blocks linked by fingerprints, checked by many nodes) is similar.
What to remember
- A blockchain is a chain of blocks, each linked to the one before it by a digital fingerprint: change an old block and the chain breaks visibly.
- Thousands of independent nodes each keep a full copy, updated in parallel, so there’s no single point of failure.
- The ledger is public: anyone can see the transactions, but not the real identity behind the addresses.