SHA-1 is a cryptographic hash function that is no longer considered secure and should only be used for backward compatibility.
In 2006, NIST issued a policy stating that:
"Federal agencies should stop using SHA-1 ... as soon as practical, and must use the SHA-2 family of hash functions for these applications after 2010."
Even with its weaknesses, SHA-1 is a significantly better hash than md5. No actual collisions have yet been found on the full SHA-1 hash. See "No SHA-1 Collision? Yet SHA1 is broken?" for more information.