What Is a Blockchain and How Does It Work?

A blockchain is a shared digital record of transactions that is copied across many independent computers and grouped into blocks, each linked to the previous one with cryptography, so past entries are extremely hard to change without everyone noticing. Instead of trusting one company to keep the books, participants follow common rules to agree on which transactions are valid and in what order.

That idea, a ledger nobody owns but everyone can verify, is what makes cryptocurrencies like Bitcoin possible. It also underpins smart contracts, stablecoins and tokens. If you understand the basic mechanics, the rest of crypto becomes much easier to follow.

The Building Blocks: Transactions, Blocks and Hashes

A transaction is an instruction such as "send 0.1 coins from address A to address B." It is signed with the sender's private key, which proves the owner approved it without revealing the key itself. Our guide to public and private keys explains how that signing works.

Transactions are collected into a block. Each block contains a batch of transactions, a timestamp and a hash of the previous block. A hash is a short digital fingerprint produced by a mathematical function: change even one character of the input and the fingerprint changes completely. Our article What Is Cryptocurrency? How Digital Money Works expands on this point.

Because each block includes the previous block's hash, the blocks form a chain. If someone tried to edit an old transaction, that block's hash would change, which would break the link to the next block, and the next, all the way to the present. Every honest node would immediately reject the altered version.

Nodes: Many Copies of the Same Ledger

A node is a computer running the blockchain's software. Full nodes download the entire history and independently check every transaction and block against the rules: Is the signature valid? Does the sender actually have these funds? Is the block correctly formed? Our article What Is Cryptocurrency? A Clear Beginner's Guide expands on this point.

Because anyone can run a node on a public blockchain, there is no single server to shut down or bribe. If one node goes offline or lies, the others simply ignore it. This is what people mean when they call a blockchain "decentralized," although the degree of decentralization varies a lot between networks. We cover this in more detail in What Is a Blockchain? A Plain-English Explanation.

Consensus: How the Network Agrees

Nodes need a way to agree on which block comes next, especially when two valid blocks appear at the same time. That process is called consensus. The two most common approaches are:

Both designs make rewriting history extremely expensive. To change past blocks, an attacker would need to control a majority of the mining power or staked value, a scenario known as a 51% attack. On large networks this is impractical; on small ones it has happened. For more detail, see proof of work explained and proof of stake explained.

How a Transaction Moves Through a Blockchain

Here is what happens when you send crypto from your wallet:

1. 1Your wallet creates the transaction and signs it with your private key.
2. 2The signed transaction is broadcast to nodes, which check it and hold it in a waiting area called the mempool.
3. 3A miner or validator selects transactions, usually favoring those that pay higher fees, and packages them into a new block.
4. 4The block is shared with the network, and nodes verify it before adding it to their copy of the ledger.
5. 5As more blocks are added on top, the transaction becomes harder to reverse. This is called confirmation, and many services wait for several confirmations before treating a payment as final.

You can watch this happen yourself using a block explorer, a website that displays the public ledger. Our guide on how to use a block explorer shows you how.

What Blockchains Are Good At (and Not)

Blockchains shine when people who don't trust each other need a shared record that nobody can secretly edit:

• Digital money that can be sent globally without a bank.
• Tokens representing assets, from stablecoins to tokenized funds.
• Smart contracts: programs that run exactly as written, used for lending, trading and more.
• Public audit trails, where transparency is the point.

They are a poor fit when a trusted operator already exists and speed matters more than openness. A traditional database is faster, cheaper and easier to fix when mistakes happen. Blockchains also have real trade-offs: limited throughput, fees that spike when demand is high, and mistakes that usually can't be reversed. Many "blockchain" projects in business never needed one. Related reading: ICO vs IEO vs IDO.

Risks and Common Misconceptions

• "On the blockchain" doesn't mean true or safe. A blockchain faithfully records whatever valid transactions it receives, including scams. A token existing on-chain says nothing about whether it is legitimate.
• Transparency is not privacy. On most blockchains, every transaction is public. Addresses are pseudonymous, but they can often be linked to real identities.
• Irreversibility cuts both ways. If you send funds to the wrong address or sign a malicious transaction, there is usually no one to call.
• Decentralization varies. Some networks are run by a handful of validators or can be paused by a foundation. Check before assuming a chain is as decentralized as Bitcoin.

Bottom line

A blockchain is a shared ledger kept by many independent computers, chained together with cryptography and updated through a consensus process that makes cheating costly. It is a powerful tool for open, verifiable record-keeping, but it does not make the projects built on it trustworthy. Before using any coin or token, check its official details and live data, for example on its page in our cryptocurrency listings, and remember that the chain will record your mistakes as faithfully as your successes.