What Is Blockchain? A Plain-English Explanation

Blockchain is often mentioned alongside cryptocurrencies, but the underlying idea is a way of recording information so that it is hard to alter after the fact. This article explains the concept without hype and without giving financial advice.

Key takeaways

• A blockchain is a ledger stored across many computers and made of linked blocks.
• Each block contains a cryptographic fingerprint of the one before it.
• Changing old records would break the chain, so tampering is easy to detect.
• Participants agree on new entries through a consensus method.
• Blockchains have trade-offs, including energy use, speed and complexity.

A shared ledger

A ledger is a record of transactions. In a traditional system, a bank or another central organisation keeps the ledger. In a blockchain, many computers, called nodes, each hold a copy. When new transactions are made, they are collected into a block and, after the network agrees it is valid, the block is added to the end of the chain.

How blocks are linked

Each block contains a batch of transactions, a timestamp and a hash of the previous block. A hash is a fixed-length string produced by a mathematical function from any input. Even a tiny change in the input produces a completely different hash. Because each block includes the hash of the one before it, altering an old block would change its hash and break every link that follows, which other nodes would notice.

Reaching agreement

With no central referee, a network needs rules for agreeing on which new block to accept. Bitcoin uses proof of work, where computers compete to solve a puzzle that takes effort to solve but is easy to check. Other networks use proof of stake, where participants lock up tokens as a stake and are chosen to validate blocks. These approaches differ in energy use, security assumptions and speed.

Public and private blockchains

Public blockchains, such as Bitcoin, allow anyone to join, read the ledger and submit transactions. Private or permissioned blockchains limit who can participate and are sometimes used by groups of organisations that want a shared record.

What blockchains can and cannot do

A blockchain can provide a record that many parties can verify without trusting a single operator. It does not by itself guarantee that the data entered is true: if false information is written in, the chain will faithfully preserve it. Blockchains are often slower and more costly than ordinary databases, so many tasks are better served by traditional tools.

Smart contracts

Some blockchains support smart contracts, programs stored on the chain that run automatically when conditions are met. Bugs in contract code can have serious consequences, so careful auditing is important.

A note of caution

Cryptocurrencies are volatile and are targeted by scams. This article is educational information only, not investment advice. Anyone considering buying digital assets should do their own research and consider speaking with a qualified financial professional.

Bitcoin's origins and proof of work

The first widely used blockchain was created for Bitcoin. In October 2008, a person or group using the name Satoshi Nakamoto published a paper describing a peer-to-peer electronic cash system, and the first block, the genesis block, was created in January 2009. Bitcoin uses proof of work: participants called miners compete to find a number that, combined with the block's data, produces a hash below a target. The winner adds the block and receives newly created coins and fees. The difficulty adjusts automatically so that blocks arrive roughly every ten minutes, and the reward is cut in half about every four years in events called halvings.

Ethereum and smart contracts

Launched in 2015, Ethereum generalised the idea by adding a programming layer. Developers can write smart contracts, programs stored on the blockchain that run exactly as written when conditions are met, and build decentralised applications on top of them. This led to new financial tools, tokens and online communities, but also to bugs and exploits, because code on a public blockchain is hard to change once deployed. In 2022, Ethereum switched from proof of work to proof of stake in an upgrade called the Merge, which cut its energy use dramatically.

Uses beyond cryptocurrency

Companies and governments have tested blockchains for supply-chain tracking, land registries, credentials and voting. Some pilots reported benefits, such as a shared record that many parties can check, but many projects were dropped because a conventional database was simpler and cheaper. Non-fungible tokens, which record ownership of a digital item, became a short-lived craze with large sales and then steep falls. The honest summary is that blockchains suit a narrow set of problems where several parties need a shared record and do not trust a single owner.

Energy and the environment

Proof of work networks consume substantial electricity, because miners run powerful machines to compete. Estimates for Bitcoin place its yearly use in the range of a mid-sized country, though figures vary and depend on the energy mix. Supporters point to use of surplus or renewable energy, while critics argue the cost is too high. Proof of stake and other designs use far less energy but have different trade-offs.

Risks and scams

The crypto world has seen many frauds and failures. Phishing sites steal wallet credentials, fake investment schemes promise guaranteed returns, and projects have collapsed when founders vanished, a practice called a rug pull. In 2022, the exchange FTX failed, and customers lost large sums. Regulators in many countries are developing rules. Never share the recovery phrase for a wallet, be sceptical of promises of high returns, and remember that this is educational information, not financial advice.

Frequently asked questions

Is blockchain the same as Bitcoin?

No. Bitcoin is a cryptocurrency that uses a blockchain. The technology can be used for other purposes.

Can a blockchain be hacked?

Large public blockchains are hard to alter, but exchanges, wallets and smart contracts built around them have been attacked.

Is blockchain anonymous?

Many blockchains are pseudonymous, meaning transactions are public but linked to addresses, not names, and addresses can sometimes be traced.

What is a crypto wallet?

A wallet is software or hardware that stores the private keys needed to access and move assets on a blockchain. The assets themselves live on the blockchain, and whoever holds the keys controls them, so losing the keys can mean losing the funds.

What is a fork?

A fork is a change in the rules of a blockchain. A soft fork tightens the rules and stays compatible with older software, while a hard fork creates incompatible rules and can split a network into two chains if not everyone agrees.

Is blockchain regulated?

Regulation varies by country and by use. Some places treat cryptocurrencies as property or securities, require exchanges to register and impose tax rules, while others restrict or ban some activities. Rules are changing, so check local guidance.

Sources and further reading

• Blockchain (Wikipedia)
• NIST: Blockchain technology overview