What is blockchain? A shared ledger copied across many computers. How blocks, hashes, and consensus work, and what blockchain can't guarantee.
Blockchain is a shared digital ledger that many computers keep identical copies of, with new records grouped into blocks and each block linked to the one before it by a cryptographic fingerprint. Because every copy can be checked against the others, quietly changing an old record becomes impractical, and no single bank, company, or government has to run the ledger.
Since Bitcoin made the idea famous in 2009, the word most often attached to it has been “immutable.” For an investor, the useful part is what that word covers and what it leaves out, because a blockchain can prove when a record was written and which key signed it, yet it cannot check whether the record was true when it went in.
In simple terms, a blockchain is a record book that many independent computers keep copies of and agree on together. New entries are added in batches called blocks, each block points back to the one before it, and the rules make old pages so hard to rewrite that the whole network can treat its shared history as settled.
In its 2018 Blockchain Technology Overview, the US National Institute of Standards and Technology (NIST) describes blockchains as tamper-evident and tamper-resistant digital ledgers, usually run without a central authority. Tamper-evident means any change would be visible to anyone who checks; tamper-resistant means making that change is very expensive. Taken together, those two properties are what people usually mean when they call a blockchain immutable.
A blockchain works by turning each transaction into a signed message, sharing it with a network of computers called nodes, bundling waiting transactions into a block, and having the network agree on which block comes next. Once a block is accepted, every node adds it to its own copy, so all of the copies keep the same history.
The steps below follow a single payment on a public blockchain such as Bitcoin:
A block is a batch of transactions plus a header (a short summary recording the time, the previous block's hash, and one hash covering every transaction inside). On Bitcoin, the target is one block every 10 minutes on average, with mining difficulty recalculated every 2,016 blocks, as the Bitcoin developer documentation explains.
A hash is a fixed-length fingerprint that a hash function, such as Bitcoin's SHA-256, produces from any input. Even a one-character change produces an unrelated result: the SHA-256 hash of “Alice pays Bob 10” begins 69ad18df, while “Alice pays Bob 11” begins 83e970b5. Since each block stores the previous block's hash, editing an old transaction breaks the link to every later block, and a forger would have to rebuild all of them faster than the honest network adds new ones.
A node is a computer that runs a blockchain's software and keeps a copy of the ledger. On a public blockchain, full nodes check every block against the network's rules and reject anything invalid; each operator can verify the whole history without asking permission.
Blockchain networks agree on the next block through a consensus mechanism, a set of rules that decides who may add a block and which version of the chain everyone follows. Of the many designs, the two most common are proof of work, which Bitcoin uses, and proof of stake, which Ethereum has used since its upgrade in September 2022.
In proof of work, miners race to find a block hash below a target number, which takes real hardware and electricity. As of Q4 2026, Bitcoin miners were producing more than 1,000 EH/s, mempool.space mining data shows. To rewrite recent history, an attacker would need more computing power than the rest of the network combined.
In proof of stake, validators lock up the network's own coin as collateral and take turns proposing and voting on blocks, and a validator caught signing two conflicting blocks loses part of its stake (a penalty called slashing). By ethereum.org's estimate, the network's energy use fell by about 99.95% when it switched to this model in the Merge on September 15, 2022.
As of Q4 2026, about 817,000 validators securing roughly 44.1 million staked ETH were active in beacon chain data compiled by validatorqueue.com. That count overstates the number of independent parties, because one operator can run many validators and, since the Pectra upgrade in May 2025, a single validator can hold up to 2,048 ETH. For the trade-offs between the two models, see our comparison of proof of work vs proof of stake.
The main types of blockchain are public, private, consortium, and hybrid, and they differ in who may add blocks. On a public, permissionless blockchain, anyone can run a node and help produce blocks. Private and consortium blockchains are permissioned, so only approved nodes add blocks, and a hybrid design keeps most records private while making selected ones public.
| Type | Who can add blocks | Who can read it | Typical use | Main trade-off |
|---|---|---|---|---|
| Public (permissionless) | Anyone who follows the rules | Anyone | Bitcoin, Ethereum, and most cryptocurrencies | Slower, but no gatekeeper |
| Private (permissioned) | One organization's approved nodes | Set by the owner | One company's internal records | Fast, but users must trust the owner |
| Consortium (permissioned) | Known member organizations | Set by the members | Banks or trading partners sharing one record | Needs agreed governance |
| Hybrid (partly permissioned) | Approved nodes | Private, with selected records public | Confidential data with public verification | More complex to govern |
Public blockchains such as Bitcoin and Ethereum let anyone read the ledger, send transactions, and, with the right hardware or stake, take part in producing blocks, which is why they need costly consensus mechanisms to keep anonymous participants honest; our guide to what permissionless means in crypto covers the trade-offs. In permissioned networks, NIST notes, members are known and misbehavior can be handled through legal remedies, which lets these networks use lighter consensus methods. Our explainer on distributed ledger technology goes deeper into ledger designs beyond blockchain.
A blockchain guarantees that a settled record was written in a particular order and has not been changed since, and anyone running a node can check that independently. It cannot guarantee that the record was true when it was entered, that the person holding a key is the rightful owner, or that a transaction is final the instant it appears.
Since a blockchain only knows what is written to it, a smart contract (a program stored on the chain) that pays out on an asset price needs an outside data feed, called an oracle. Ethereum's developer documentation calls this the oracle problem: a contract is only as correct as its data feed, and trusting the oracle's operator brings back the middleman the blockchain was meant to remove.
Finality is the point after which a transaction cannot be reversed. On Bitcoin it is probabilistic, and bitcoin.org's guidance for new users lists six confirmations (roughly an hour) as the minimum for high-value transfers. Ethereum finalizes blocks at checkpoints once validators holding two-thirds of staked ETH agree. In a research note on single-slot finality, Ethereum co-founder Vitalik Buterin put the wait at 64 to 95 slots (roughly 13 to 19 minutes), and reversing a finalized block would cost an attacker at least one-third of all staked ETH.
Large public blockchains are expensive to attack, because rewriting their history requires a majority of the network's computing power or stake. Even so, smaller networks have been overpowered, and the largest crypto losses tend to come from the systems built around a chain, through stolen private keys, hacked platforms, and flawed smart contracts that the blockchain then records faithfully.
A 51% attack happens when one party controls most of a network's mining power or stake and rewrites recent blocks, usually to spend the same coins twice. In August 2020, Ethereum Classic, a smaller proof-of-work network, suffered three such attacks in a single month. During the first, an attacker double-spent more than 800,000 ETC (about $5.6 million at the time), CoinDesk reported.
In its December 2025 review of stolen funds, Chainalysis estimated that over $3.4 billion in crypto was stolen from January through early December 2025, with the largest losses coming from attacks on private keys and signing systems. Because a transfer signed with a stolen key settles like a legitimate one, your safety depends far more on how keys are stored than on the chain's cryptography.
Blockchain is used mainly for cryptocurrencies, stablecoins, and smart contracts that run lending, trading, and other financial services without a central operator. Beyond crypto, businesses use permissioned blockchains to share records with partners, and a growing area called tokenization records ownership of traditional assets, such as bonds or fund shares, as tokens on a blockchain.
Bitcoin was the first application, and it is still a payment network secured by its own blockchain. Stablecoins (tokens designed to hold a steady value, usually one US dollar) have since become one of the largest uses by value: DefiLlama's stablecoin dashboard puts the combined market value of dollar-pegged stablecoins at roughly $300 billion as of Q4 2026.
Ethereum was designed largely to host smart contracts, and the lending and trading services built from them are known as decentralized finance (DeFi); our explainer on how smart contracts work covers the bugs and oracle failures that can break them.
Outside DeFi, companies use permissioned blockchains to share one record with partners, such as banks reconciling payments. With tokenization, the same idea is applied to ownership, so a fund share or a bond can be recorded as a token and moved between holders.
A blockchain is the right tool only when several parties must write to the same record, they do not trust each other, and no trusted operator can run the record for them. If any condition fails, an ordinary database is usually the better fit and runs faster, as computer scientists Karl Wüst and Arthur Gervais concluded in 2018.
The four-question blockchain test below adapts the decision flowchart in “Do you need a Blockchain?”, their 2018 paper. Work through it in order and stop at the first destination.
| Question | If yes | If no |
|---|---|---|
| 1. Do several parties need to write to the same record? | Go to question 2 | Use a database |
| 2. Can everyone rely on one trusted operator that is always online? | Let that operator run a database | Go to question 3 |
| 3. Are all the writers known in advance? | Go to question 4 | A public, permissionless blockchain |
| 4. Do the writers trust one another? | A shared database | A permissioned blockchain |
For a currency meant for anyone, anywhere, the test never points to a database, because the writers are unknown and have no referee they all accept. By contrast, a company tracking its own inventory (a single writer) stops at the first question.
A traditional database is run by one administrator who can add, edit, or delete records, so it is fast but asks users to trust that administrator. In a blockchain, edits become visible or impractical, and the design trades speed for a history anyone can verify; Wüst and Gervais note that a centralized system generally beats a blockchain on latency and throughput.
No. Blockchain is the record-keeping technology, while Bitcoin, launched in January 2009 as the first of its kind, is a digital currency running on its own blockchain. Many other cryptocurrencies, stablecoins, and business systems run on blockchains of their own, so Bitcoin is one application of the technology and the best-known example of it.
The pseudonymous Satoshi Nakamoto built the first working blockchain for Bitcoin, describing it in a 2008 white paper and mining the first block on January 3, 2009. Chaining time-stamped records with hashes is an older idea, and Nakamoto's paper cites 1991 research by Stuart Haber and Scott Stornetta.
Generally, no: once a transaction is buried under enough blocks, or finalized on a proof-of-stake network, no bank or administrator can pull it back. Bitcoin.org notes that a Bitcoin payment can only be refunded by the person who received it, so coins sent to a wrong address usually cannot be recovered unless the recipient returns them.
Yes. On most public blockchains, such as Bitcoin, every transaction and address balance is visible to anyone. In NIST's terms, users are pseudonymous, meaning names are not attached to addresses but all of an address's activity can be seen. Once funds pass through a platform that verifies its customers, investigators can often link an address to a real person.
A blockchain wallet is software or a device that stores the private keys controlling your addresses, while the coins themselves always stay on the blockchain. Whoever holds the keys can move the funds, and custody (who holds those keys) comes in three broad forms: a platform's own books, a qualified third-party custodian, or a wallet you control yourself.
Blockchain is a technology, so it cannot be bought directly; investors seeking exposure usually buy a network's native asset, such as BTC or ETH, or shares in companies that use it. Each carries its own risks, and a useful technology does not guarantee any asset's price. None of this is a recommendation to buy or avoid any asset.
Knowing how a blockchain settles and secures transactions makes it easier to ask a broker good questions about where your own assets are held. UpTrade is a dedicated crypto brokerage built around real relationships, not a self-serve app you navigate alone.
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