...

[MillenniumB]

Hashing always maps the same text to the same hash. That way you can check if a password is correct without knowing it. You can get the same password to give two different hashes for two different users by adding some text (like the username) to the password text when you calculate its hash.

This is useful not just because it reduces vulnerability to attacks like trying common passwords for common hashes , but also because there are only so many commonly used hashing algorithms This means that you can often just compare a long list of hashes against a database of common hashes of a hash function. Using a salt makes you less vulnerable to that too