Secure Hash Algorithm 2 is a set of cryptographic hash functions designed by the United States National Security Agency (NSA).
The SHA-2 family consists of multiple closely related hash functions. It is essentially a single algorithm in which a few minor parameters are different among the variants. The initial spec covered four variants with output sizes of 224, 256, 384 and 512 bits. The most significant difference between the variants is that some are designed for 32 bit registers and some are designed for 64 bit registers. In terms of performance this is the only difference that matters.
Cryptographic hash functions are mathematical operations run on digital data; by comparing the computed “hash” (the output from execution of the algorithm) to a known and expected hash value, a person can determine the data’s integrity. For example, computing the hash of a downloaded file and comparing the result to a previously published hash result can show whether the download has been modified or tampered with
In simple terms, hashing means taking an input string of any length and giving out an output of a fixed length. In the context of cryptocurrencies like Bitcoin, the transactions are taken as an input and run through a hashing algorithm (Bitcoin uses SHA-256) which gives an output of a fixed length.