What is a blockchain?

The short answer

A blockchain is a database that is shared across many computers, where new entries are added in batches called blocks and each block is cryptographically linked to the one before it. Once a block is accepted by the network, changing it would require changing every block after it on most of the computers at once, which is designed to be impractical. That is what makes the record hard to tamper with.

The key idea is that no single organization is in charge. Instead of a bank keeping the only copy of a ledger, thousands of independent participants each keep a full copy and follow the same rules to decide what gets added. They do not need to trust each other, because the rules and the math do the work that trust usually does.

Bitcoin was the first widely used blockchain, launched in 2009 to track who owns which coins. Since then the same structure has been used for programmable networks like Ethereum and Solana, where the ledger records not only balances but the state of running applications. The structure is the same; what changes is the kind of data being recorded.

Blocks, chains, and hashes

A block is just a bundle of data, usually a list of transactions plus a small header. The header contains a fingerprint of the previous block, produced by a mathematical function called a hash. A hash takes any input and produces a fixed length string of characters. Change even one character of the input and the output changes completely.

Because each block contains the hash of the block before it, the blocks form a chain where every link depends on all the earlier ones. If someone edited a transaction in an old block, that block's hash would change, which would break the link stored in the next block, and so on all the way to the present. Everyone else's copy would immediately reveal the mismatch.

Each block also has a position in the sequence, known as its height, and a timestamp. Blockchains differ in how often blocks are produced. Bitcoin targets one block every ten minutes, Ethereum produces one roughly every twelve seconds, and some newer networks produce several per second. Faster blocks mean quicker confirmations but usually require more powerful nodes.

Nodes and consensus

The computers that store and verify the blockchain are called nodes. A full node downloads every block, checks that every transaction follows the rules, and rejects anything invalid. Because every node checks independently, a bad block cannot spread even if the person who created it is powerful or well funded.

Consensus is the process by which nodes agree on which new block to add next. Different blockchains use different methods. Proof of work, used by Bitcoin, has miners compete to solve a computational puzzle. Proof of stake, used by Ethereum since 2022 and by Solana, Cardano, and many others, selects validators who have locked up coins as collateral.

Whatever the method, the goal is the same: make it expensive to cheat and cheap to verify. Attacking a proof of work chain means outspending the entire network on hardware and electricity. Attacking a proof of stake chain means risking the loss of a huge amount of staked coins. Honest participation, by contrast, is rewarded with new coins and fees.

Why blockchains matter

A blockchain allows strangers anywhere in the world to agree on a shared record without a trusted middleman. For money, that means payments that cannot be reversed or censored by a company. For software, it means applications that keep running exactly as written even if their creators disappear. For records, it means a history that anyone can audit.

This property enables things that were previously difficult. Stablecoins move dollars around the world in minutes. Decentralized exchanges let people trade without handing their funds to a custodian. Tokens can represent ownership of art, shares in a fund, or membership in an organization, with the ownership record maintained publicly. None of this requires permission from a bank or registrar.

Blockchains are also transparent. Anyone can look up any transaction that has ever happened on Bitcoin or Ethereum using a block explorer. That openness is useful for auditing and research, though it also means privacy requires extra care, because addresses can sometimes be linked to real identities. Several networks and tools exist specifically to add privacy on top of public chains.

• Agreement among strangers without a central authority
• Records that are very hard to alter after the fact
• Applications that run as written without an operator
• Full public transparency of the transaction history

Limitations and tradeoffs

Decentralization has a cost. Because every node processes every transaction, a blockchain can handle far fewer transactions per second than a centralized database. Bitcoin processes a handful per second on its base layer, and even fast chains are slower than a major payment processor. Layer 2 networks and other scaling techniques exist to address this.

Blockchains are also only as good as the data put into them. A blockchain can guarantee that a record has not been changed, but it cannot guarantee the record was true in the first place. This is why using blockchains for real world items like supply chain tracking is harder than it sounds, and why many corporate blockchain projects of the late 2010s were quietly abandoned.

Finally, irreversibility cuts both ways. A payment that cannot be reversed by a bank also cannot be reversed if you were scammed or made a typo. Public blockchains put responsibility on the user in a way that most people are not used to. A private or permissioned blockchain, run by a known group of companies, gives up most of the trustless benefits in exchange for control.

What to watch as you learn

When you encounter a new blockchain, ask a few basic questions. How does it reach consensus, and who can participate? How many independent nodes or validators does it have? How much does a transaction cost, and how long until it is final? What has its uptime record been? The answers tell you more than marketing claims about speed.

Try looking up a real transaction on a block explorer such as mempool.space for Bitcoin or Etherscan for Ethereum. Seeing a block, its hash, the link to the previous block, and the list of transactions inside it makes the abstract idea concrete. You can trace the chain back block by block to the very first one.

From here, the natural next steps are to learn how proof of work and proof of stake differ, how wallets and private keys give you control over what is recorded, and how smart contracts turn a ledger into a platform for applications. Each of those topics has its own explainer here, and they build on the foundation you now have.