What is blockchain? It is a shared record book, called a ledger, that many computers keep in identical copies and that can only be extended by adding new pages, never by quietly rewriting old ones. Each page, called a block, holds a batch of transactions and is linked to the one before it, which is why the whole thing is called a chain. This design lets strangers agree on who owns what without a bank or any other central operator in the middle, and it is the technology behind Bitcoin, Ethereum and most other cryptocurrencies.
In brief
- A blockchain is a ledger copied across many computers, so no single company or person has to be trusted to keep it honest.
- Transactions are grouped into blocks, and each block carries a digital fingerprint of the previous one, so changing the past breaks every later link.
- Computers called nodes follow shared rules, known as a consensus mechanism, to agree on which new block is valid.
- Public blockchains are open to read and hard to alter, but they can be slow, costly to use and they do not protect you from scams or from losing your keys.
- You do not need to understand the mathematics to use crypto, but you should understand what the chain does and what it does not do.
In this article · 12 min read
- What is blockchain, in one sentence
- How a blockchain works, step by step
- The problem blockchain was built to solve
- Consensus: how strangers agree
- A number worth knowing: the block reward halves
- Public, private and other kinds of blockchain
- Blockchain, Bitcoin and smart contracts
- Advantages and limits
- Where it fits for your own crypto
- How to start learning, step by step
- Checklist
- Who it is for
- Key terms
- Frequently asked questions
- Sources
What is blockchain, in one sentence
Blockchain is a type of database in which records are stored in linked blocks and copied across a network, so that the history is hard to change without everyone noticing. The US National Institute of Standards and Technology (NIST) describes blockchains in its Blockchain Technology Overview, published in October 2018, as tamper evident and tamper resistant digital ledgers implemented in a distributed fashion, usually without a central authority. "Tamper evident" means that if someone alters a record, the change can be detected. "Distributed" means that many participants hold a copy instead of one owner holding the original.
The idea was put to work in 2008, when a person or group using the name Satoshi Nakamoto published the Bitcoin white paper. It proposed a system of electronic payments that does not rely on a trusted third party, and it used a chain of blocks, each linked to the previous one, to timestamp transactions. In his book Cryptocurrency Investing, John Bax describes the Bitcoin blockchain as a decentralised piece of software that records, with certainty, every transaction made on the network.
How a blockchain works, step by step
The details differ from one blockchain to another, but the basic path of a transaction is similar on most public networks:
- Someone sends a transaction. The sender signs it with a private key, a secret code that proves control of an address. The public key and address are the part you can share, a bit like an account number.
- Nodes check it. A node is a computer that runs the network software and keeps a copy of the ledger. Each node verifies that the signature is valid and that the coins have not already been spent.
- Transactions wait to be bundled. Valid transactions gather in a pool, and a block is assembled from some of them.
- The network agrees on the next block. This is where the consensus mechanism comes in, the set of rules that lets thousands of independent computers settle on one version of the truth.
- The block is linked to the chain. It carries a hash of the previous block. A hash is a fixed-length fingerprint calculated from data: change one character of the data and the fingerprint changes completely.
- Everyone updates their copy. Once the block is accepted, the transaction becomes part of the shared history.
Because every block contains the fingerprint of the one before, altering an old transaction would change its block's hash, then the next block's reference, and so on up to the present. To get away with it, an attacker would have to redo all that work and convince the network to accept the new version. The Bitcoin white paper makes the same point about the cost of rewriting a chain.
The problem blockchain was built to solve
Digital files can be copied for free, which is a problem for money: if a coin is just a file, what stops someone from spending it twice? This is called double spending. Until Bitcoin, the usual answer was to put a trusted intermediary, such as a bank or a payment company, in charge of the ledger. The Bitcoin white paper proposed a different answer: let a network of computers keep the ledger together, and make the order of transactions public and verifiable.
John Bax explains in Cryptocurrency Investing that this is precisely why the blockchain records all transactions that have ever occurred: with no controller, something has to guarantee that nobody spends the same bitcoin twice, and a public, ordered, practically unchangeable record does that job.
Consensus: how strangers agree
A blockchain needs a rule for deciding who gets to add the next block, because everyone could otherwise add their own. The two best-known rules are these:
- Proof of work. Miners compete to solve a computing puzzle, and the winner adds the block and earns a reward. Bitcoin uses it. It makes rewriting history expensive, but it consumes a lot of electricity. The Bitcoin network is designed to produce a block about every ten minutes.
- Proof of stake. Validators lock up coins as a deposit and are chosen to propose blocks; dishonest behaviour can cost them part of that deposit. On Ethereum, according to ethereum.org, a new slot, the opportunity to propose a block, comes every twelve seconds. Ethereum switched from proof of work to proof of stake in 2022, and the Crypto Carbon Ratings Institute estimates, as reported on ethereum.org, that this change cut its annualised electricity consumption by more than 99.988%.
Neither system is perfect. The Bank for International Settlements, in its Annual Economic Report of June 2018, examined what it called the "elusive promise of decentralised trust" and noted that good money needs to scale efficiently with the economy. These are fair limits to keep in mind.
A number worth knowing: the block reward halves
Blockchain rules are written in code, and Bitcoin's rules include a schedule for creating new coins. The reward paid to the miner of each block started at 50 bitcoin in 2009 and is cut in half every 210,000 blocks, roughly every four years. According to the Bitcoin Wiki's table on controlled supply, the reward fell to 25 bitcoin in November 2012, to 12.5 in July 2016, to 6.25 in May 2020 and to 3.125 in April 2024. Measured against the original 50 bitcoin, the reward is now 6.25% of its starting size, as the chart shows.
This is an example of what a blockchain does well: a rule that everyone can read and nobody can change at will. In our experience, readers who see this schedule for the first time understand much better why people say Bitcoin has a fixed design rather than an operator.
Blockchain 101: a visual demo (Anders Brownworth) (in English)
Public, private and other kinds of blockchain
| Type | Who can read | Who can add blocks | Typical use |
|---|---|---|---|
| Public | Anyone | Anyone who follows the rules | Bitcoin, Ethereum |
| Private (permissioned) | Approved members | Approved members | Company or consortium records |
| Hybrid | Depends on the design | Depends on the design | Mixed projects |
Public blockchains are the ones that matter for crypto. In Cryptocurrency Investing, John Bax stresses a point that surprises many beginners: because the Bitcoin blockchain is public and every past movement can be traced, Bitcoin is pseudonymous, not anonymous. Addresses are not names, but once someone converts coins to regular money on an exchange that checks identity, the trail can lead back to a person. In his view, this makes Bitcoin a poor tool for criminals, whatever the myth says.
John Bax's view
John Bax also treats decentralisation as a scale rather than a yes-or-no label. In his reading, Bitcoin sits at the top of that scale and many other projects sit well below it. Ethereum applications, for example, are open to everyone ("permissionless"), but the websites most people use to reach them are run by private companies and can be switched off. His practical lesson is to compare assets only within the same category and not to assume that "on a blockchain" means "decentralised".
Blockchain, Bitcoin and smart contracts
Blockchain and Bitcoin are not the same thing. Bitcoin is the first cryptocurrency, and the blockchain is the data structure it runs on. Other networks use the same idea differently. Ethereum added smart contracts, programs stored on the blockchain that run automatically when their conditions are met. They power decentralised finance, or DeFi, a set of financial services built without a traditional intermediary. Our guide on what DeFi is covers the benefits and the risks. A stablecoin, a token designed to track a currency such as the US dollar, also lives on a blockchain, as we explain in our stablecoins guide.
Advantages and limits
Blockchain has real strengths, and also limits that marketing often skips:
- Transparency. On public chains, anyone can inspect the ledger. The BSI, a national cybersecurity agency, lists tamper-resistant data and transparent, traceable transactions among the main aims of the technology.
- Resilience. There is no single server to switch off, because many nodes hold the same data.
- Costs and speed. Depending on the network and how busy it is, fees can rise and confirmations can be slow. The BSI also notes that proof of work uses a lot of energy per transaction.
- Not a shield against scams. A transaction written to the chain is very hard to reverse, so a payment sent to a fraudster is usually gone. Hacks and fake apps remain a major risk, as our guide on crypto scams shows.
- Not the whole of crypto. Trading on an exchange happens in the exchange's own system, not on the blockchain. Only deposits, withdrawals and on-chain activity touch the chain.
Supervisors also remind consumers that the technology does not remove market risk. The European Supervisory Authorities (EBA, ESMA and EIOPA) warned in 2022 that many crypto-assets are highly risky and speculative and that you can lose all the money you invest.
Where it fits for your own crypto
Understanding the chain helps with practical choices. Your coins live at an address on the blockchain, and the key to that address is yours only if you control it, which is why a seed phrase matters so much. If you are new, our guide on how to buy cryptocurrency walks through a first purchase. John Bax's Cryptocurrency Investing covers the blockchain, wallets and exchanges in a few plain chapters before it moves on to investing.
Automated trading is a different layer. A trading bot normally sends orders to an exchange, and those orders are not recorded on the blockchain. If you are curious about that world, see how a crypto trading bot works, and note that Crypto Go Bot is a trading tool, not a way to avoid risk.
How to start learning, step by step
- Read about one blockchain at a time. Start with Bitcoin, then look at Ethereum.
- Look at a block explorer. It is a website that shows blocks and transactions live, which makes the idea concrete.
- Learn what a wallet and a seed phrase are before putting money anywhere.
- Start with a small amount you can afford to lose, and use well-known platforms only.
- Be careful with offers. Anyone promising certain returns because it is "on the blockchain" is a warning sign.
Checklist
- I can explain a block, a node and a hash in one sentence each.
- I know that a public blockchain shows addresses, not names.
- I know that trades on an exchange are not always on the chain.
- I keep my seed phrase offline and never type it into a website.
- I do not trust a project only because it uses blockchain.
Who it is for
Blockchain is worth understanding for anyone who touches crypto, and for some people who do not.
- Complete beginner about to buy a first coin: knowing what a wallet, an address and a confirmation are makes the first transfer much less stressful.
- Long-term holder: understanding how supply rules and security work helps you judge a project's claims over a horizon of several years.
- Curious professional: if you work in banking, logistics or software and keep hearing about blockchain, the basics show where it truly fits and where an ordinary database works better.
- Active trader: knowing the difference between on-chain activity and exchange trading explains fees, delays and why deposits take time.
Key terms
- Blockchain: a ledger stored as linked blocks and copied across a network of computers.
- Node: a computer that runs the network software and keeps a copy of the ledger.
- Hash: a fixed-length digital fingerprint calculated from data, which changes completely if the data changes.
- Consensus mechanism: the set of rules that lets the network agree on the next valid block.
- Proof of work: a consensus method in which miners spend computing power to earn the right to add a block.
- Smart contract: a program stored on a blockchain that runs automatically when set conditions are met.
Frequently asked questions
What is blockchain in simple words?
It is a shared notebook of transactions that many computers keep in identical copies. New entries are added in blocks that are linked to the previous ones, so changing old entries is very hard to do unnoticed.
How does blockchain work?
A user signs a transaction, nodes check it, and the network agrees on a new block that contains it. Each block holds the hash of the previous one, which links the blocks together and makes tampering visible.
Is blockchain the same as Bitcoin?
No. Bitcoin is a cryptocurrency, and the blockchain is the technology it uses to record transactions. Many other cryptocurrencies and applications use their own blockchains.
Is blockchain safe and can it be hacked?
The cryptography behind well-designed chains is widely tested, but risk remains around them. Exchanges, wallets, smart contracts and people are the usual targets, so protecting your keys and avoiding unknown platforms matters more than the chain itself.
Is blockchain anonymous?
Usually not. On public chains, addresses and transactions are visible, and they are pseudonymous rather than anonymous. A link to a real identity can appear, for example, when coins are exchanged through a service that verifies identity.
Do I need blockchain to invest in crypto?
You use it indirectly whenever you send or receive coins, but you do not need to understand every technical detail. It helps to know the basics so that you can judge claims and protect your assets.
Can a blockchain be changed?
Old records are designed to be extremely hard to alter, because that would mean redoing the work for every later block and convincing the network. The rules of a blockchain can still change if its community agrees on an upgrade.
Sources
- Wikipedia, Blockchain
- NIST, Blockchain Technology Overview (NISTIR 8202), October 2018
- Satoshi Nakamoto, Bitcoin: A Peer-to-Peer Electronic Cash System, 2008
- Bank for International Settlements, Annual Economic Report 2018, chapter V: Cryptocurrencies: looking beyond the hype, 24 June 2018
- ethereum.org, Blocks and Ethereum energy consumption
- Bitcoin Wiki, Controlled supply
- BSI (Germany), Blockchain & Kryptowährung
- ESAs (EBA, ESMA, EIOPA), Warning on the risks of crypto-assets, 2022
