# What is a blockchain?

> A plain-English explanation of what a blockchain actually is — a shared, append-only ledger that no single party controls.

Before diving into QRL's quantum-resistant cryptography, it helps to be clear on what a blockchain actually is. Strip away the hype and it's a surprisingly simple idea.

## A shared ledger with no owner

A **ledger** is just a record of transactions — who paid whom, and when. A normal ledger lives with one trusted party: your bank keeps the record of your account.

A **blockchain** is a ledger that is **copied across thousands of independent computers**, with no single owner. They all hold the same history and follow the same rules to keep it in sync. There is no central operator to trust — or to hack, censor, or shut down.

The hard problem this creates is agreement: how do thousands of strangers settle on one shared history? The answer is the **[consensus mechanism](/learn/consensus-pow-vs-pos)**, which we cover separately.

## Why "block" and why "chain"

Transactions aren't written one at a time. They're collected into **[blocks](/learn/blocks-and-transactions)** — batches confirmed together at a regular rhythm. Each block carries a **[hash](/learn/glossary#hash-function)** (a cryptographic fingerprint) of the block before it.

```
[ Block 100 ] → [ Block 101 ] → [ Block 102 ] → …
   hash of 99      hash of 100     hash of 101
```

That backward link is the "chain." It's also the source of the blockchain's tamper-resistance: change anything in an old block and its hash changes, which breaks the link in every block that follows. To get away with it you'd have to re-make all those blocks faster than the rest of the network makes new ones — which the consensus rules are designed to make absurdly expensive.

## What lives on a blockchain

At minimum, a blockchain records **balances** and **transfers** of its native coin. Many chains also run **programs** — smart contracts — that move value under coded rules. This is one place QRL's two chains differ: QRL 2.0 supports [smart contracts](/learn/smart-contracts-on-qrl2), while QRL Legacy supports only a fixed set of transaction types.

Crucially, the ledger is **append-only and public**. Anyone can read it, and once something is confirmed and buried under enough later blocks, it is effectively permanent. That's exactly what a block explorer like this one lets you browse.

## Where QRL fits

QRL is a blockchain with one distinguishing feature: the **signatures** that authorise its transactions are [quantum-resistant](/learn/quantum-resistance-explained), unlike the ECDSA most chains use. Everything else on this page — blocks, chaining, a shared ledger, consensus — still applies. QRL just swaps the cryptography that protects your funds for schemes a quantum computer can't break.

## Where to go next

- [Blocks and transactions](/learn/blocks-and-transactions) — what's actually inside a block.
- [Wallets, addresses and keys](/learn/wallets-addresses-and-keys) — how ownership works.
- [Proof-of-Work vs Proof-of-Stake](/learn/consensus-pow-vs-pos) — how blocks get agreed on.