sha1 is what is called a "one way encryption". what this means is that it can be encrypted, but there is no decryption.
sha1 encryption is used in security for storing encrypted passwords. if the user where to forget there password, the only option is to generate a new one, even the site admin will not be able to determine the password from the encrypted value stored in the database.
see the following for information on various encrypting techniques:
encryption_1/