SHA-2 refers to a family of cryptographic hash functions that can convert arbitrarily long strings of data into digests of a fixed size. This digest is commonly displayed as a fixed value hexadecimal number. (SHA-256, for instance, returns a 64 character code.)
SHA-2 has supplanted SHA-1 in security protocols like SSL/TLS.
The Secure Hash Algorithm (SHA) series of hash functions is comprised of different sets (SHA-0, SHA-1, SHA-2, SHA-3).