Secure Hash Algorithm (SHA-1) computes a fixed-length digital representation (known as a message digest) of an input data sequence (the message) of any length.

SHA-1 is employed in several widely used security applications and protocols, including TLS and SSL, PGP, SSH, S/MIME, and IPsec. It was considered to be the successor to MD5, an earlier, widely-used hash function.

Also see: Wikipedia