What is a blockchain?

A blockchain is a digital record book maintained by multiple participants. New records are organized into blocks, and the blocks are connected in order. Computers participating in the blockchain network check updates under the same rules, so one database administrator does not have sole authority to decide which records are valid.

More precisely, a blockchain combines a data structure, cryptography, replicated copies of data and consensus rules. These features make changes to earlier records easier to detect and, while the network's security assumptions hold, increasingly difficult to rewrite. They do not make rewriting impossible under every circumstance.

A blockchain is therefore more than a database containing hashes. It combines independently applied validation rules with a process for deciding which valid version of the record participants will follow.

Why use a blockchain instead of one database?

A conventional database and a blockchain can both store and retrieve data. The main difference is who maintains the records and how participants decide which version is valid.

An ordinary database is usually managed by one organization. Its administrator decides who can read or change data and can correct, delete or restore records directly. Users mainly rely on that organization and its systems.

A blockchain instead allows multiple nodes—computers running the network software—to check updates under shared rules and keep their own copies of the ledger. A transaction or block proposed by one node does not become valid automatically. Other nodes check details such as signatures, balances and transaction format, and the network uses consensus and branch-selection rules to decide which history to continue following.

The two approaches suit different needs:

• An ordinary database is usually faster, less expensive and easier to edit or keep private. It suits activities managed by one trusted organization.
• A blockchain can suit participants who need a shared record but do not want one participant to decide the result alone. The trade-off is additional computation, communication and storage, which usually reduce efficiency.

A blockchain also does not mean that everyone can do everything. Permissionless networks generally allow anyone to participate under the protocol rules, while permissioned networks restrict certain activities to approved organizations. In both cases, access and influence may still be uneven.

Four tasks that maintain the ledger

Transactions or state updates

Users or applications submit signed transactions or other updates defined by the protocol. Nodes check authorization, format, balances, transaction sequence numbers, spending conditions and other applicable rules. Broadcasting a message does not make it valid.

Blocks and cryptographic commitments

Valid updates are grouped into blocks. A block normally uses hashes, including tree-shaped hash summaries of multiple records, to make a cryptographic commitment to the previous block and its own contents. If earlier data changes, later commitments no longer match unless the affected history is rebuilt. This makes tampering detectable, but it does not by itself decide which competing history the network accepts.

Replication and independent validation

Nodes exchange blocks and keep the data required for their roles. A fully validating node applies the protocol rules itself instead of relying on a block explorer or another node's description. Not every participant stores the entire history or performs the same duties, and “distributed” does not mean that resources or influence are evenly distributed.

Consensus and branch selection

Consensus coordinates the accepted order of valid proposals. Bitcoin full nodes validate blocks, while proof of work provides a chain-selection signal based on accumulated work. In Ethereum's proof-of-stake system, validators vote on block history: a branch-selection rule determines which branch to follow, and checkpoints with enough votes can reach protocol-defined finality. Different consensus mechanisms rely on different assumptions and should not all be described as mining.

What changes when a transaction is accepted?

Bitcoin uses an unspent transaction output (UTXO) model. A UTXO is an amount created by an earlier transaction that has not yet been spent. A valid transaction consumes existing outputs under their spending conditions and creates new ones; nodes reject conflicting spends in the accepted history. Ethereum instead maintains account-based state. Its transactions can update balances, transaction sequence numbers, contract code or storage under protocol rules.

In either model, the blockchain is not a folder that moves coin files between addresses. Nodes compute which state follows from the accepted sequence of valid updates. A wallet prepares and signs instructions or interacts with a provider; the wallet interface is not the shared ledger itself.

Tamper evidence, reorganizations and finality

Cryptographic links make a changed historical block inconsistent with later commitments, but hashes do not physically prevent rewriting. Resistance comes from the cost and decision rules required to replace the accepted history, together with participants applying the validation rules correctly.

Recent history can be reorganized when the network's branch-selection rule chooses another valid branch. Some protocols provide increasing confidence as more blocks accumulate; others define explicit finality through votes or certificates. These guarantees can still fail under severe faults, protocol bugs or the compromise of enough participants. Describing every blockchain record as permanently immutable hides these conditions.

What consensus does not prove

Consensus can show that network participants accepted data under the protocol rules. It cannot independently prove that a statement about the outside world is true. If a data provider reports a temperature or a system records that a parcel arrived, putting that value onchain proves the record and its processing history—not the physical event itself.

The same distinction applies to code. Deterministic execution can reproduce what a smart contract was instructed to do, but it does not guarantee that the code has no defect, that an interface explained the call correctly or that the economic outcome is desirable.

Blockchain, cryptocurrency and distributed ledger

A blockchain is a distributed-ledger design that organizes records into linked blocks. A cryptocurrency is an asset accounted for under a network's rules. Distributed ledger technology (DLT) is a broader category and does not always use one chain of blocks. Some distributed ledgers use a directed acyclic graph (DAG), in which records form a graph with no circular path instead of one sequence of blocks. Not every distributed ledger is therefore a blockchain.

A blockchain can support purposes beyond transferable assets, but using the structure does not automatically make an application decentralized, private, scalable or trustworthy. Those properties require separate evidence about governance, node operation, data visibility and protocol design.

FAQ

Does every blockchain use mining?

No. Mining is associated with proof-of-work systems. Proof-of-stake networks use validators and votes, while permissioned systems may use other consensus protocols.

Is a blockchain the same as one block?

No. A block is one protocol-defined data unit. A blockchain is the linked ledger history maintained under network rules.

Can data ever be removed from a blockchain?

The answer depends on what “removed” means and on the protocol. A reorganization can displace recent blocks; applications may stop displaying data; some systems permit state changes or pruning. Historical commitments and copies held by participants may remain. “Delete,” “prune,” “revert” and “become non-canonical” are different operations.

Related concepts and further reading

Read about blocks, blockchain addresses, crypto wallets and gas fees, or return to the Crypto Glossary.

For a broader historical comparison, see OKX Learn's overview of blockchains, Bitcoin and Ethereum. It is supplementary reading rather than a protocol specification.

Sources

• NISTIR 8202: Blockchain Technology Overview — distributed-ledger architecture, consensus, permission models, tamper resistance and external-data limitations.
• Bitcoin Developer Guide: block chain — full-node validation, UTXOs, proof of work and chain selection.
• Bitcoin Developer Reference: block chain — block and transaction data structures.
• Ethereum: proof-of-stake — validators, attestations, fork choice and finality.
• Ethereum: accounts — account-based state, EOAs and contract accounts.