Blocks, Hashes & Chains

Each block contains a hash of the previous block, forming an immutable chain. A block typically includes: a header (with the previous block's hash, timestamp, nonce, and Merkle root), and a body (containing the list of transactions). The hash function (SHA-256 in Bitcoin) takes all block data as input and produces a fixed-length output. Even a tiny change in any data produces a completely different hash, making tampering immediately detectable. The chain of hashes linking blocks together is what gives 'blockchain' its name and its security guarantees.