What is a hash function?
A hash function takes an arbitrary string as input then it produces an output that is fixed in length called Message Digest or Hash Value or just Hash.
So, SHA-256 is a hash function that always produces 256 bit long output.
What is SHA?
SHA stands for “Secure Hash Algorithm” is an algorithm to produce hash output from an input.
Its enhanced version is called SHA-1. The algorithm offers five separate hash functions which were created by National Security Agency (NSA) and were issued by the National Institute of Standards and Technology (NIST).
MD4, MD5, SHA-256, SHA-512
An n-bit hash is a map from arbitrary length messages to n-bit hash values.
Current popular hashes produce hash values of length n = 128 (MD4 and MD5) and n = 160 (SHA-1)
An n- bit cryptographic hash is an n-bit hash which is one-way and col lision-resistant. Therefore above hashes can provide no more than 64 or 80 bits of security, respectively, against collision attacks.
SHA-256 is a 256-bit hash and is meant to provide 128 bits of security against collision attacks.
SHA-512 is a 512-bit hash, and is meant to provide 256 bits of security against collision attacks.