SHA-256 processes 64-byte blocks through 64 rounds over 32-bit words in a Merkle–Damgård construction and outputs 32 bytes. Its digest of the empty string is e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 and of abc is ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad. No attack comes close to its security bounds.
Most modern CPUs have SHA-256 instructions (Intel and AMD SHA extensions, ARMv8 crypto extensions). On one core of an Apple M4 Max, OpenSSL 3.6.5 hashes about 2.85 GB/s with 16 KiB buffers, faster than SHA-512 (1.65 GB/s) and SHA3-256 (1.01 GB/s) on the same machine. Bitcoin applies it twice (SHA-256d) for block and transaction ids.
Its Merkle–Damgård structure allows length extension, so SHA-256(secret ‖ message) is not a safe MAC; use HMAC-SHA256. Identifiers: OID 2.16.840.1.101.3.4.2.1, JOSE S256 in PKCE and sha-256 in SD-JWT's _sd_alg, SHA-256 in WebCrypto, sha256 in OpenSSL and Node.