What is a blockchain? A shared record many computers keep, and hard to change

Yahoo Finance, in an explainer dated April 22, 2026, describes a blockchain as a distributed ledger that records what happened and when. A U.S. News explainer calls it a digital ledger of transactions spread across a large network of computers. No single bank keeps the only copy. Changing an old entry is hard.

Key facts, per U.S. News, Yahoo Finance and MIT Technology Review (2018 page)

• What it is: A digital ledger spread across a large network of computers.
• Who keeps it: Every node on the network holds a copy, not one ledger kept by a central authority such as a bank.
• Changing an old block: Tampering would mean repeating the proof of work for every later block.
• Limit: Control of at least 51% of the network gives control of consensus and the ledger.

Who holds the record

Plasma's educational guide (accessed Oct. 15, 2026) says many computers keep the record in sync, so no single company controls it. The guide is the company's own page. Yahoo Finance adds that every node holds a copy of the ledger. It describes nodes as computers running specialized software that interacts with the blockchain.

How one transaction is recorded

MIT Technology Review's explainer, dated April 23, 2018, walks through one transaction step by step. Nodes validate a transaction when it follows the network's rules. Valid transactions are gathered into a block, which holds a cryptographic reference to the block before it. Yahoo Finance adds that blocks are stacked in chronological order, forming a chain.

How the network agrees

U.S. News describes two ways to validate transactions. Under proof of work, miners use powerful computers to complete hashes, which are complex mathematical functions. Under proof of stake, validators are chosen by how much cryptocurrency they have staked. It calls that approach more energy-efficient.

Yahoo Finance says Ethereum used proof of stake after its 2022 upgrade, an approach based on collateral. It adds that proof of stake can cut energy use by as much as 99% compared with proof of work. Some security trade-offs apply.

Why the record is hard to change

Plasma's guide says that once a block is added, altering it would mean rewriting every later block on most of the network at once. MIT Technology Review describes the same effect under proof of work: tampering with an earlier block would mean repeating the work for every later block. Yahoo Finance says that once data is recorded, it is extremely difficult to alter.

The limit: 51% control

U.S. News spells out the limit. If one party gains control of at least 51% of the network, it controls consensus and can manipulate the ledger. Hard to change is not the same as impossible to change. The 51% line marks where that protection stops.

Public, private and consortium chains

U.S. News says Bitcoin was the first public blockchain and ranks among the largest. A private blockchain needs an invitation. Its entries are validated by the business that created it, or by a set of rules that business put in place. Yahoo Finance places consortium blockchains between public and private.

Beyond payments: smart contracts

Yahoo Finance describes smart contracts as programs stored on a blockchain. They run agreements automatically once set conditions are met, with no third party acting as referee.

Where the idea came from

Plasma's guide says blockchain became famous through Bitcoin, which a paper by someone using the name Satoshi Nakamoto described in 2008. According to MIT Technology Review, the first bitcoins were mined in January 2009 by Nakamoto. Three dated steps trace that path.

How fast it runs

Yahoo Finance says Bitcoin adds a new block once every 10 minutes on average. It puts Bitcoin at about seven transactions per second.

Analysis: where trust sits

Across these sources, trust moves from one institution's ledger to rules that many computers apply. The types show where it stops. A private chain follows rules its creator sets. The 51% line shows that anyone holding at least 51% of the network can manipulate the ledger.

What is confirmed and what is not

Confirmed by two or more outlets: the ledger is copied across many computers (U.S. News, Yahoo Finance, Plasma), and changing an earlier block means redoing the work for every later block (MIT Technology Review, Plasma). That explainer dates from 2018 and frames its mechanics around proof of work. The speed and energy figures are Yahoo Finance's own, and no other source here gives a figure for them. To check them, look for the methodology behind them in a primary source before relying on them. A separate explainer covers how custodial and non-custodial wallets divide control of your keys.

Sources: Blockchain Definition (money.usnews.com), accessed Oct 15, 2026 · Yahoo Finance, accessed Oct 15, 2026 · Explainer: What is a blockchain? (technologyreview.com), accessed Oct 15, 2026 · What Is Blockchain? A Simple Guide for Beginners (plasma.org), accessed Oct 15, 2026

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