What is a blockchain?
The blockchain is the public ledger that records every Bitcoin transaction ever made. Understanding it explains why Bitcoin doesn't need a bank, and why you can trust it without trusting any specific person or institution.
What a "block" actually is
Roughly every 10 minutes, a new block is added to the Bitcoin blockchain. Each block is a bundle of transactions that were broadcast to the network during that time window, think of it like a page in an accounting ledger.
Every block contains:
- A list of transactions (who sent Bitcoin to whom, and how much)
- A timestamp
- A cryptographic reference (called a "hash") to the previous block
- A proof of work that miners competed to generate
Why the chain matters
That reference to the previous block is what makes this a chain. Each new block is mathematically linked to every block before it. If you tried to go back and alter an old transaction, you'd break the chain, the hashes would no longer match, and the entire network would reject your version.
This is what makes the blockchain immutable. You'd need to redo the cryptographic work for that block and every block after it, faster than the rest of the network is adding new ones. For Bitcoin, that's computationally impossible at any realistic cost.
Who maintains it?
No single entity. Thousands of computers around the world (called nodes) each keep a complete copy of the blockchain. They constantly cross-check each other. For a fraudulent transaction to be accepted, you'd need to control a majority of the network's computing power simultaneously, an attack that would cost billions of dollars and would likely be detected and countered before it succeeded.
Why this matters for your money
When your Bitcoin is recorded on the blockchain, that record belongs to no bank and no government. It can't be frozen, reversed, or confiscated by a third party, as long as you control your private key.
The blockchain is the guarantee. Your ownership is enforced by math, not by trust in any institution.