What Is a Block in a Blockchain? The Page Where Transactions Are Written

A block is one page of a blockchain's record book. It holds a batch of recent transactions, the time they were written, and a fingerprint of the page before it. Once the network agrees the page is valid, it is added to the end of the chain and everyone's copy grows by one. Bitcoin fills a new page roughly every ten minutes. Ethereum does it every twelve seconds.

What is actually inside a block?

Three things, in plain terms. A list of transactions — who paid whom, and how much. A short summary of the block that came before it, which is what links the pages into a chain. And a header with the timestamp and the proof that the block was produced properly. Bitcoin blocks are small on purpose, holding a couple of thousand payments each, which is why space in them is worth paying for.

Why can't someone rewrite an old block?

Because every page carries a fingerprint of the one before it. Think of a stack of numbered receipts where each receipt copies the total from the last. Change an amount halfway down and every receipt above it stops adding up, in a way anybody can spot in seconds. To make the tampered version stick, an attacker would have to redo the work behind every block since — while the rest of the network keeps building on the honest one.

Why does the block matter when you send money?

Your payment is not final the moment you press send. It waits, then a miner or validator picks it up and includes it in the next block. Only then does it exist on the chain. Each block built on top afterwards is one more confirmation that it will not be undone, which is why exchanges make you wait for a few before crediting a deposit.

Something to try: paste any transaction into a block explorer and look at the block number it landed in, then watch the number of confirmations climb. It makes the whole thing concrete far faster than any diagram, and it is the same view a node operator sees.