A block is a bundled “package” of data added to a blockchain. Most commonly, it contains a batch of transactions that have been validated by the network, plus metadata that helps the chain stay ordered and tamper resistant.
What a block contains and how it links the chain
Blocks are the building units of a blockchain ledger. Alongside transaction records, a block typically includes a timestamp, a reference to the previous block (often called the previous block hash), and a unique identifier derived from its contents. That cryptographic reference is what “chains” blocks together. If someone tries to alter a past transaction, the block’s identifier changes, which breaks the link to every block that follows and makes the tampering obvious to network participants.
Different blockchains store slightly different fields, but the core idea is consistent: a block summarizes a set of state changes, then anchors them to the history that came before.
How blocks are created and confirmed
New blocks are proposed by participants running the network’s consensus mechanism. In proof of work systems, miners compete to produce a valid block according to protocol rules. In proof of stake systems, validators are selected to propose and attest to blocks. Once the network accepts a block, its transactions are considered confirmed. As more blocks are added on top, it becomes progressively harder to reorganize or replace that history.
A practical way to see this is with a block explorer. When you look up a transaction, you can usually view the block that included it, the block’s height (its position in the chain), and other details showing how it connects to neighboring blocks.
Why blocks matter in crypto
Blocks determine how transaction history is recorded, verified, and secured without a central administrator. Understanding blocks helps explain confirmations, network security, and why blockchains can function as shared, auditable ledgers in the crypto ecosystem.