Block Header is the precisely serialized structure whose double SHA-256 is the Bitcoin block hash. It contains neither the transaction list nor proof of every transaction’s validity.
Fields are version (4 bytes), previous-header hash (32), Merkle root (32), time (4), nBits (4) and nonce (4): 80 total. Numeric fields use little-endian; hashes serialize in internal byte order, different from the usual hexadecimal identifier display. [Bitcoin Core v29.0: header serialization]
nBits compactly encodes the target, not the explorer’s difficulty number. Nodes check hash at most equal to target and the correct target for the height. Time must exceed the median of the previous eleven blocks and pass the future-time check; it is neither trusted payment time nor exact mining duration. [Bitcoin Core v29.0: header validation]
Changing nonce changes the hash directly; changing coinbase changes Merkle root and thus the header. Header chains support checks of linkage and work. Scripts, spent outputs, rewards and other content rules still require transaction data and state context. [Bitcoin Core v29.0: header validation]
For the clearest picture, read this entry together with Block, Proof of Work, Merkle tree, Nonce. The reverse links also lead from Block, Cryptographic hash, SHA-256, Merkle tree.