SHA-256 (Secure Hash Algorithm)
SHA-256 (Secure Hash Algorithm, FIPS 182-2 ), is one of the cryptographic hash function which has digest length of 256 bits. It’s a keyless hash function, means an MDC (Manipulation Detection Code). In other words, SHA (Secure Hash Algorithm) was developed by the National Institute of Standards & Technology, and further, they came with a new version called SHA-256 (the SHA-2 family), where the number is represented as the hash length in bits..
Theis quite like SHA-1 structure. For SHA-256, the message is padded & divided into 512-bit blocks, the same way as for SHA-1.
SHA-256 is one of the six variants of SHA family (SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, SHA-512/256). These variants differ in size of their output, internal state size, block size, message size & rounds.
These days, all the major SSL Certificate issuers use SHA-256 which is more secure and trustworthy. Before SHA-1 was used, but it has been deprecated in January 2016, due to its security vulnerabilities it has become more susceptible to attacks and one of the reasons was a smaller bit size.