Block Header

A block header is the compact summary at the top of every block: typically the hash of the previous block, a Merkle root committing to all transactions inside, a timestamp, and consensus data such as a difficulty target and nonce or a validator signature. It is only a few dozen bytes, but it is cryptographically bound to the block's full contents, so changing any transaction changes the header's hash. That property is what lets light clients follow a chain by downloading headers alone and verifying inclusion proofs, rather than the whole ledger. Block explorers show header fields such as the parent hash and Merkle root, which is how you confirm that two nodes are on the same chain.