A blockchain is a chain of records linked by cryptographic fingerprints. Try to change one of the old blocks below and watch the entire chain after it break. That tiny demo is the whole reason blockchain works as a tamper-proof ledger.
Try to tamper with a block
BLOCK #001
Prev hash
000000...0000
Transactions
Genesis
This block hash
a3f81b...
BLOCK #002
Prev hash
a3f81b...
Transactions
Alice → Bob: 0.5 BTC
This block hash
9c2e74...
BLOCK #003
Prev hash
9c2e74...
Transactions
Bob → Carol: 0.2 BTC
This block hash
5b817f...
BLOCK #004
Prev hash
5b817f...
Transactions
Carol → Dave: 0.1 BTC
This block hash
e8a30c...
BLOCK #005
Prev hash
e8a30c...
Transactions
Dave → Eve: 0.05 BTC
This block hash
f1d942...
Click any block
Each block stores a hash - a fingerprint computed from its contents and the previous block's hash. Click any block above to try to alter it.
Section 01 · Quick answer
What it is, how it links, why it matters
What it is
A shared list of records grouped into blocks. Thousands of computers worldwide keep a synchronized copy. No single owner, no central server.
How it links
Each block carries the hash (fingerprint) of the previous block. Change one old block and every block after it points to nothing. The chain breaks visibly.
Why it matters
First time in history a permanent record can exist without anyone in charge of it. The technology underneath Bitcoin, Ethereum and every other crypto.
Section 02 · 60-second definition
Blockchain in 60 seconds
A blockchain is a database that grows by appending blocks. Each block is a fixed-size container of transactions or records. Blocks are produced one at a time, in order, on a schedule. Bitcoin produces a new block every 10 minutes. Ethereum every 12 seconds. Solana every 0.4 seconds.
The crucial trick: each block carries the hash of the previous block. A hash is a short fingerprint computed from the block's contents - change a single character of the data and the hash changes completely. Because every block points back to the previous one's fingerprint, the chain is built like a stack where each layer depends on the one below it.
The hero above shows this. Click any past block - the simulator marks it as tampered, and every block after it turns red because the linkage is broken. In a real network, the other nodes would reject the broken chain and ignore the change.
Section 03 · How a block is born
How a new block gets added
Step 1. Users send transactions. Each transaction broadcasts to the network and waits in a memory pool (mempool) until a block has space.
Step 2. A node (called a miner on Bitcoin, validator on Ethereum) bundles a few thousand pending transactions into a candidate block. It computes the block's hash, including the previous block's hash inside it.
Step 3. The node proves it has the right to add the block - either by burning energy (Bitcoin's proof-of-work) or by staking coins as collateral (Ethereum's proof-of-stake). The proof prevents anyone from cheaply spamming the network with fake blocks.
Step 4. The new block broadcasts. Every other node verifies it independently and adds it to their copy of the chain. The chain just grew by one. Process repeats forever.
Section 04 · Public vs private
Public vs private blockchains
Two flavors exist. Most of what you hear about (Bitcoin, Ethereum) is public. Private chains exist mostly inside enterprises and rarely matter for end users.
Public blockchain
Anyone can read, anyone can write (by paying the fee), thousands of independent nodes verify. Genuinely tamper-resistant because no single party can control the majority of nodes.
Examples: Bitcoin, Ethereum, Solana, Avalanche
Private blockchain
Operated by a single company or consortium. Permission required to read or write. Faster and cheaper but trust depends on the operator. The operator can edit history if it wants.
Examples: Hyperledger Fabric, Corda, internal bank chains
Section 05 · Misconceptions
Four things people get wrong
Myth
"Blockchain is the same as Bitcoin"
Truth
Bitcoin is one cryptocurrency. Blockchain is the data structure underneath it. Thousands of other blockchains exist, each hosting its own coins and applications. Read more in our guide on cryptocurrency.
Myth
"Blockchain is fully anonymous"
Truth
It is pseudonymous. Every transaction is permanently public and tied to an address. Once an address is linked to a real identity (through an exchange, for example), the entire history becomes traceable.
Myth
"Blockchain is too slow for real use"
Truth
Bitcoin is slow on purpose. Modern chains run thousands of transactions per second. Solana settles 50,000+ TPS; layer-2 rollups on Ethereum hit similar numbers. The "slow" critique is stuck in 2015.
Myth
"Once on the blockchain, it is gone forever"
Truth
Half right. The data is permanent and public. But coins can be moved if you have the private key. Lose the key and the coins are gone too - they sit on the chain forever, unmovable. This is why storage matters.