Imagine you want to send $50 to a friend in another country. Today, you rely on a chain of institutions - your bank, their bank, possibly a network like SWIFT in between - to agree that you had the money, that you sent it, and that they received it. You’re trusting strangers to keep an accurate record on your behalf.

That trust is expensive. It takes time, fees, and a heavy infrastructure of compliance and reconciliation. And it can fail: banks freeze accounts, intermediaries make errors, and in some parts of the world, reliable banking infrastructure simply doesn’t exist.

Blockchain was invented to solve a specific version of this problem: how do you get a group of people who don’t know or trust each other to agree on a shared record, without any single party being in charge?

The ledger at the centre

At its core, a blockchain is a ledger - a record of transactions. But unlike a traditional ledger kept by a single entity (a bank, a company, a government), a blockchain ledger is copied across thousands of computers simultaneously. There is no single master copy. Every participant holds the same version.

When someone wants to add a new entry - say, “Alice sent 1 BTC to Bob” - they broadcast that transaction to the network. Participants (called nodes) check whether Alice actually has the funds to send, and if everything checks out, the transaction is queued to be written into the ledger permanently.

Blocks and the chain

Transactions don’t get added to the ledger one by one. Instead, they’re bundled together into blocks. A block is just a batch of verified transactions, along with some metadata.

What makes this a chain is how those blocks link together. Each block contains a piece of data called a hash - a unique fingerprint of that block’s contents, generated by a cryptographic algorithm. Each block also contains the hash of the previous block.

This creates a chain of references stretching all the way back to the very first block (called the genesis block). If you tried to alter a transaction buried three blocks back, you’d change that block’s hash, which would break the link to the next block, which would then break its link to the block after that, and so on. The entire chain after your edit would become invalid.

To tamper successfully, you’d have to redo the computational work for every subsequent block, faster than the entire honest network is adding new ones. On a large, well-distributed network, that is effectively impossible.

What hashing actually does

A hash function takes any input - a word, a file, an entire block of transaction data - and produces a fixed-length string of characters that looks like random noise. The same input always produces the same hash. But change even a single character of the input and the output hash changes completely.

This is why hashes are useful as fingerprints. If you know the hash of a block, you can verify at any time that the block’s contents haven’t been tampered with. Any modification, however small, produces a completely different hash.

Reaching agreement: consensus

For the ledger to work, all participants need to agree on which blocks are valid and in what order they were added. This is solved by a consensus mechanism.

Bitcoin uses Proof of Work: to add a new block, a participant (called a miner) must solve a computationally expensive puzzle. Solving it requires real energy and hardware investment, which means that attacking the network costs something real. The longest valid chain wins, and honest miners are rewarded for their work with newly issued Bitcoin.

Ethereum switched to Proof of Stake in 2022: instead of burning electricity, participants lock up (stake) their own cryptocurrency as collateral. If they try to cheat, they lose their stake. Honest participation is financially rewarded; dishonest participation is financially punished.

Different blockchains use different consensus mechanisms, each with different tradeoffs around security, speed, and decentralisation.

What this enables

Once you have a shared, tamper-resistant ledger with no single owner, several things become possible that weren’t before:

  • Permissionless transfers: anyone with an internet connection can send value to anyone else without needing a bank account or institutional approval.
  • Transparency: on public blockchains, every transaction is visible to anyone who wants to look. The ledger is auditable.
  • Programmability: blockchains like Ethereum can run smart contracts - code that executes automatically when certain conditions are met, without needing a middleman to enforce the agreement.

What it isn’t

Blockchain is not magic and it is not a solution to every problem. It introduces real costs: energy consumption, slower transaction speeds than centralised systems, complexity, and the irreversibility of errors (there is no customer service line to call if you send funds to the wrong address).

It is also not inherently private. Most public blockchains are pseudonymous at best - your wallet address is visible, even if your legal name is not attached to it.

The technology is a tool. Like any tool, its value depends entirely on whether it’s the right fit for the problem at hand.

This article is part of Chain 101 - our plain-English series covering crypto fundamentals.