A block is a container for transactions. Miners or validators gather pending transactions, verify them, and bundle them into a block. Each block has a header and a body. The header holds the previous block's hash, a timestamp, and a Merkle root that summarizes all the transactions inside. The body holds the actual list of transfers. Once a block is validated and added, the transactions inside it are confirmed.

The hash is the key. It is a fixed-length fingerprint of the block's data. Change one digit anywhere and the hash changes completely. Each block includes the hash of the block before it, which links them into a chain. To alter an old transaction, an attacker would need to redo the work for that block and every block after it. That is why deep blocks are considered irreversible.

What goes into a block

• Block header — version, previous hash, Merkle root, timestamp, difficulty target, nonce
• Transaction list — sender, receiver, amount, signatures
• Coinbase transaction — the first transaction, which pays the miner

Bitcoin blocks are limited to roughly 1 megabyte of data, which caps throughput. Other chains use larger blocks or different structures to process more transactions per second.