How blockchain works

Blockchain is a shared, append-only ledger copied across many independent computers that agree on its contents through consensus. So what is blockchain in plain terms? It is a database no single party owns, where once a transaction is written and confirmed, rewriting it becomes cryptographically and economically impractical. That's the whole trick, and everything below explains how it holds.

How a blockchain actually works

Picture a spreadsheet that thousands of computers keep at once. When someone makes a transaction, it gets broadcast to the network, bundled with others into a block, and every participant checks that it follows the rules. Once the network agrees, the block is added to the end of the chain, and each computer updates its own copy. No head office signs off. The agreement is the authority.

Those computers are called nodes, and the honest ones are the point. A full node stores the entire ledger and independently verifies every rule, so it doesn't have to trust anyone else's word. Ownership works differently too. You don't log in with a username and password. You control funds with a private key held in a crypto wallet, and whoever holds the key holds the asset. That design trades speed for trust on purpose, since every node re-runs the same work to stay in agreement, and you accept the cost for what it buys in return.

Blocks, hashing, and why the chain is hard to rewrite

Each block holds a batch of transactions plus a cryptographic hash of the block before it. A hash is a fixed-length fingerprint of some data. Feed it the same input and you always get the same output. Change a single character and the output looks completely different.

That linking is what makes the ledger tamper-evident. Say you edit a transaction buried ten blocks deep. Its hash changes, no longer matches the fingerprint recorded in the next block, and breaks every link after it, all the way to the tip. The network sees a copy that does not add up and rejects it. To make the edit stick, you'd have to recompute every block from your change to the present, faster than the rest of the network extends the honest chain. On a large network that isn't realistic, which is why people call blockchains immutable, meaning immutable in practice, not by magic.

Consensus: proof of work vs proof of stake

Consensus is the rule that decides which block the network accepts next, and it is where the security actually lives. Two mechanisms dominate.

• Proof of Work (PoW) — miners race to solve a hard math puzzle, and the first to solve it proposes the next block and earns a reward. Solving costs real electricity, so the honest chain is simply the one with the most accumulated work. Bitcoin runs this way, producing a block roughly every 10 minutes, with a reward of 3.125 BTC per block since the April 2024 halving.
• Proof of Stake (PoS) picks validators based on collateral, not electricity. To take part you lock up the network's token as a bond, get chosen to propose or attest to blocks, and lose part of your stake if you try to cheat. Ethereum switched from PoW to PoS in September 2022, an event called the Merge, which cut its energy use by about 99.95%.

Neither is strictly better. Proof of Work has the longest track record and a brutally simple security story, but it burns a lot of energy. Proof of Stake is far greener and settles faster, though critics argue it can concentrate influence among the largest stakers. You'll also meet variants like delegated Proof of Stake, where token holders vote for a small set of validators, and Byzantine fault tolerant schemes common in permissioned and Cosmos-based chains.

Public vs private blockchains

Not every blockchain is open to the world, and the split matters more than most newcomers expect.

A public, permissionless blockchain lets anyone join. You can run a node, read the full ledger, and send transactions without asking permission or proving who you are. Bitcoin and Ethereum work this way. Openness is the source of their censorship resistance, and also of their performance limits.

A private, permissioned blockchain restricts who can participate. A known group of organizations controls who validates and who can read what. Enterprises reach for this when they want a shared, tamper-evident record among partners without exposing it to the open internet. Hyperledger Fabric, common in supply-chain and banking consortia, is the usual example. Worth asking, though: if a small group already controls access, do you need a blockchain at all, or would a well-run shared database do the same job with less overhead? Often it would.

What blockchains are good and bad at

Blockchains earn their keep when several parties who do not fully trust each other need one shared source of truth with no referee in the middle. Settlement between institutions, provenance tracking, and programmable smart contracts all fit that shape. So does moving value across borders without a bank in the loop, which is the foundation under most DeFi.

They are a bad fit when you control all the data yourself, need high throughput, or want the option to delete records. Bitcoin handles only a handful of transactions per second and Ethereum's base layer around 15 to 20, while a payment network like Visa is built for thousands. Every node re-executing the same work is exactly why. If a plain database would serve your use case, it's almost always faster, cheaper, and simpler. Reach for a blockchain only when removing the trusted middleman is the actual requirement, not the buzzword.

Layers and networks

Choosing where to build is mostly a balance of cost, finality, security, and ecosystem, and the layer model shapes that choice.

Layer 1 networks such as Bitcoin and Ethereum provide the base security and settle transactions directly on-chain. They are the bedrock, but block space is scarce and fees rise with demand. Layer 2 networks sit on top, batching many transactions off-chain and posting compressed proofs back down to Layer 1. That buys much higher throughput and far lower fees while still inheriting the base layer's security, which is why most everyday Ethereum activity has moved to Layer 2 rollups.

Common misconceptions

A few myths cause most of the confusion, so worth clearing them up.

• "Blockchain equals Bitcoin." Bitcoin was first, but it's one network among thousands. Blockchain is the underlying pattern.
• "It's completely anonymous." Most public chains are pseudonymous, not anonymous. Your name isn't attached, but every transaction is permanently visible and often traceable back to a real identity through an exchange.
• "Data on a blockchain cannot be wrong." Immutability protects a record after it's written. It does nothing to guarantee the record was true going in. Garbage in still means garbage, permanently.
• "It's free and instant." Public networks charge fees and take time to confirm, and both can climb sharply during congestion.

Strip away the hype and blockchain is a specific tool for a specific job: keeping a shared record honest when no one is in charge. When that's your problem, few things do it better. When it isn't, a boring database usually wins.