What is a blockchain?
The most important question in crypto — answered clearly, without jargon, at three levels of depth.
Last verified: April 2026A blockchain is a shared record book that thousands of computers keep simultaneously — and once something is written in it, nobody can change or delete it.
The problem it was built to solve
When you send money to someone using a bank, you are trusting that bank to update two records correctly: subtract from your account, add to theirs. The bank controls that record. If the bank makes a mistake, gets hacked, goes bankrupt, or decides to freeze your account — there is very little you can do about it.
This is the fundamental problem with centralised record-keeping. One entity controls the truth. Everyone else has to trust them.
A blockchain solves this by distributing the record across thousands of computers simultaneously. Nobody controls it. Nobody can change it unilaterally. And anyone can verify it themselves.
The shared notebook analogy
Imagine a notebook where every financial transaction in a town is recorded. Right now, that notebook lives in one bank. The bank controls it, can edit it, and can refuse to show it to you.
Now imagine that notebook is copied across 10,000 computers worldwide, updated simultaneously every time a transaction happens, and that every copy is constantly verified against all the others. If someone tries to alter one copy, the other 9,999 copies reject it instantly. The truth is whatever all the copies agree on — and changing that would require controlling more than half of those computers at once.
That is a blockchain. The notebook is public. Nobody owns it. And it cannot be lied to.
Why is it called a "blockchain"?
Transactions are grouped into batches called blocks. Each block contains a list of recent transactions, a timestamp, and a mathematical fingerprint (called a hash) of the previous block. This fingerprint is what creates the "chain" — because each block references the one before it, going all the way back to the very first block (called the genesis block).
If you tried to change a transaction in an old block, you would change its fingerprint. That would invalidate the next block, which would invalidate the one after that, all the way to the present. Every computer on the network would immediately reject your version.
This is what makes blockchain records permanent. Not because they are locked away — because they are mathematically interlocked.
What actually lives on a blockchain?
Originally, just financial transactions: "Alice sent 1 Bitcoin to Bob at 3:42pm on this date." That is all Bitcoin's blockchain does — record transfers of value between addresses.
But Ethereum showed that you could put more than transactions on a blockchain. You can put code — programs called smart contracts — that run automatically when conditions are met. This opened up an entirely new world: decentralised finance, digital ownership, automated agreements, and more.
What a blockchain is NOT
A blockchain is not a cryptocurrency. A cryptocurrency is one thing that uses a blockchain. A blockchain can also store ownership records, votes, medical records, supply chain data, and almost anything else that benefits from a permanent, shared, tamper-proof record.
A blockchain is also not the same as a database. A database is controlled by whoever owns it. A blockchain is controlled by the rules written into its code — and changing those rules requires the agreement of the network's participants.
Sending money internationally without a bank (Bitcoin). Running financial applications without a company in the middle (Ethereum DeFi). Proving ownership of digital items (NFTs). Tracking goods through a supply chain so every participant can verify the record. Issuing government-backed digital currencies (CBDCs like India's e-Rupee).
The structure of a block
Every block in a blockchain contains four key pieces of information. The block header contains: a reference to the previous block (its hash), a timestamp, a difficulty target, and a nonce (a number miners adjust to solve the proof-of-work puzzle). The block body contains the actual list of transactions included in that block.
When a new block is added, every node on the network downloads it, verifies every transaction in it, and updates their local copy of the chain. If the block is valid, it is added. If not, it is rejected. No central authority decides — every node applies the same rules independently and arrives at the same answer.
How transactions become permanent: the confirmation process
When you send a transaction, it is broadcast to the network and enters a waiting area called the mempool (memory pool). Miners or validators pick transactions from the mempool and include them in the next block. Once your transaction is included in a block, it has one confirmation. Each subsequent block added after it adds another confirmation.
More confirmations mean more security. To reverse a confirmed transaction, an attacker would need to rebuild the blockchain from that point forward faster than the honest network — which becomes exponentially harder with each confirmation. This is why exchanges typically require 3–6 confirmations before crediting a large Bitcoin deposit.
Types of blockchain: public, private, and consortium
Not all blockchains are the same. Public blockchains (Bitcoin, Ethereum) are open to anyone — anyone can read them, anyone can transact on them, anyone can run a node. Private blockchains are controlled by a single organisation — used by companies for internal record-keeping where they want immutability without public access. Consortium blockchains are shared between a defined group of organisations — used in banking, supply chain, and healthcare where multiple parties need a shared record but do not want it fully public.
Most of what people discuss in crypto refers to public, permissionless blockchains. These are the ones where no entity controls participation.
Key metrics for assessing a blockchain's health: hash rate or validator count (security), transactions per second (TPS) (throughput), active addresses (usage), time to finality (how long until a transaction is truly irreversible), and node count (decentralisation). No single metric tells the full story — a fast chain with few validators may be faster but less decentralised.
The trilemma: security, scalability, decentralisation
Every blockchain designer faces a fundamental trade-off known as the blockchain trilemma. A blockchain can optimise for any two of the following — but doing all three simultaneously is extremely difficult: security (resistant to attacks), scalability (handles many transactions quickly and cheaply), and decentralisation (no single point of control or failure).
Bitcoin prioritises security and decentralisation at the expense of speed — it processes about 7 transactions per second. Solana prioritises security and scalability — it processes tens of thousands per second, but with a much smaller, less distributed validator set. Ethereum's answer is to use Layer 2 solutions to handle scalability off-chain while maintaining security and decentralisation on the base layer.
How blockchains differ from each other
The major differences between blockchains come down to their consensus mechanism (how they achieve agreement), their architecture (UTXO model vs account model), their programmability (Bitcoin's limited Script vs Ethereum's full EVM), their block time (how quickly new blocks are added), and their governance (how protocol changes are decided and implemented).
Understanding these differences is why the consensus mechanism page and the individual ecosystem pages matter — each blockchain made deliberate trade-offs, and those trade-offs determine what it is good for.
The cryptographic foundation
A blockchain's immutability rests on cryptographic hash functions. A hash function takes any input and produces a fixed-length output (the hash or digest). Critically, it has three properties: it is deterministic (same input always produces same output), it is one-way (you cannot reverse the hash to find the input), and it is collision-resistant (it is computationally infeasible to find two different inputs that produce the same hash).
Bitcoin uses SHA-256 (Secure Hash Algorithm 256-bit). Ethereum uses Keccak-256. Each block header contains the hash of the previous block's header. This creates the cryptographic chain — changing any block changes its hash, which invalidates every subsequent block's "previous hash" reference. An attacker would need to recompute the entire chain from the altered block forward, faster than the honest network adds new blocks.
Source: Nakamoto, S. (2008). Bitcoin: A Peer-to-Peer Electronic Cash System. Section 2: Transactions. bitcoin.org/bitcoin.pdf
Merkle trees: efficient transaction verification
A block can contain thousands of transactions. To verify that a specific transaction is in a block without downloading the entire block, Bitcoin and Ethereum use a data structure called a Merkle tree (also called a hash tree, after Ralph Merkle who described it in his 1979 PhD thesis).
Transactions are hashed in pairs, and those hashes are hashed together, repeatedly, until a single root hash (the Merkle root) represents the entire set of transactions. The Merkle root is stored in the block header. A lightweight client can verify that a transaction is included in a block with only a logarithmic number of hashes — O(log n) — rather than downloading all n transactions. This enables Simple Payment Verification (SPV), described in section 8 of the Bitcoin whitepaper.
Bitcoin's block header is 80 bytes and contains: version (4 bytes), previous block hash (32 bytes), Merkle root (32 bytes), timestamp (4 bytes), difficulty bits (4 bytes), nonce (4 bytes). Ethereum's block header is substantially larger and includes the state root (a Merkle Patricia Trie root representing the entire world state), receipts root, logs bloom filter, and additional fields not present in Bitcoin's UTXO-based architecture.
The Byzantine Generals Problem
The theoretical foundation for blockchain consensus is the Byzantine Generals Problem, formalised by Lamport, Shostak, and Pease in their 1982 paper. The problem asks: how can distributed parties reach consensus when some participants may be unreliable or malicious (Byzantine faults), and communication is not guaranteed?
Satoshi Nakamoto's key contribution was solving a practical version of this problem at internet scale, without requiring participants to know or trust each other, using computational proof-of-work as the mechanism for reaching consensus. Prior theoretical solutions required knowing the total number of participants — Bitcoin's design works with an unknown, open participant set.
Sources: Lamport, Shostak, Pease (1982). The Byzantine Generals Problem. ACM Transactions on Programming Languages and Systems. · Nakamoto (2008), Section 11: Calculations.
State and state transitions
A blockchain can be understood as a state machine. The state is the current snapshot of all accounts and their balances (or, in Ethereum, all accounts and their associated storage). Each block represents a batch of state transitions — transactions that change the state. The blockchain is the complete history of state transitions from the genesis block to the present.
Bitcoin's state is the UTXO set — all unspent transaction outputs. Ethereum's state is the world state — a mapping of every account address to its balance, nonce, code, and storage, represented as a Merkle Patricia Trie. Every Ethereum block includes a state root — the root hash of the entire world state after processing all transactions in that block — allowing any node to verify the complete state from any block.
Source: Wood, G. (2014). Ethereum Yellow Paper: Ethereum: A Secure Decentralised Generalised Transaction Ledger. ethereum.github.io/yellowpaper/paper.pdf