They can't be "decrypted" as such. The hash destroys the original information. The only way to crack hashed passwords is to use a brute force attack, either a dictionary, or random pass generator where every entry is hashed and compared against the original. There's little difference between cracking SHA-1 and MD5 in that case, but as I mentioned, cracking say, 8 character alpahnumeric passwords can take forever, but some people have databases, where every possible hash for an md5 of say, 8 character alohanumeric characters is stored, and it's much quicker to look up the hash in the database than to try and crack it with an alogorithm.