What is password salting? What attack does it help to deter?

Password salting is the processing of prepending or appending a series of characters to a user's password before hashing this new combined value. This value should be different for every single user but the same salt should be applied to the same user password every time it is validated.

This ensures that users that have the same password will still have very different hash values stored in the password database. This process specifically helps deter rainbow table attacks since a new rainbow table would need to be computed for every single user in the database.