Hash is the output of a hash function. Identical bytes and algorithm yield the same digest; cryptographic designs aim to make finding an input or different inputs with equal outputs difficult. It is not an encrypted copy.
Anyone can hash public data without a secret key. Matching a digest obtained through a trusted channel helps check integrity, but the hash alone identifies neither author nor truth. If an attacker replaces both file and digest, simple comparison does not establish origin. [NIST FIPS 180-4: Secure Hash Standard]
Preimage means finding an input for a specified digest; collision means finding any two different inputs sharing one digest. Finite output space means collisions exist; security rests on difficulty of finding them, not mathematical impossibility. Short or predictable inputs can be tested with a dictionary. [NIST FIPS 180-4: Secure Hash Standard]
Header hashes, txid and witness commitments are not interchangeable. They use specific serialization and constructions; other parts use HASH160 or tagged hashes. Hexadecimal text is only a representation: hashing hex characters instead of decoded bytes usually produces a different digest. [Bitcoin Core v29.0: hash constructions]
For the clearest picture, read this entry together with SHA-256, Proof of Work, Block header, Transaction, Merkle tree, Bitcoin. The reverse links also lead from SHA-256, Hashcash, Entropy mixing, BIP39 checksum.