http://tcllib.sourceforge.net/doc/sha1.html
Cryptographically secure hash designed by NIST and the NSA for the Secure Hash Standard. SHA yields a 160 bit result [What is a secure hash?]
For what purposes is sha1 used??
SHA is based upon the MD4 algorithm designed by Prof. R Rivest. Soon after the release of MD4 some weaknesses were found. While these have not been extended to break the full algorithm MD4 was enhanced by Rivest to create MD5. SHA is also an enhanced MD4 - but has different enhancements. These are an expand transformation, and extra round, and a faster avalanche effect. There are no known attacks against SHA. [BOOK: Applied Cryptography]
SHA might be used for any protocol where a secure one-way cryptographic hash is required. There are lots of such protocols in the literature.
See also md5.