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chakribobby, unix-style authentication using the passwd (and shadow) file works by putting the user's password through a cryptographic hashing function, and storing this hashed result. When the user tries to authenticate, the plaintext password they enter is put through the same hashing function, and if the result matches the stored value, the user is authenticated.
The hashing function is "one way", meaning that it is not possible (or infeasibly difficult) go from the hashes password back to the plain text password.
As a system administrator you should not have to reverse a password. If you want to reset a users password, this is possible, and you have to tell the user that you have reset their password (or at least they will know it because their old password will stop working). As root, you can assume the identiy of the user without having to know their password using the su program as Disillusionist mentioned.
The only legitimate reason for trying to determine a user's password is to discover weak passwords (those which can easily be guessed), which might constitute a security threat to the system you are administrating. I believe it is unethical to do this without telling users what you are doing because users often use the same password on many systems, and might not want you to know it. Of course this is bad security practise on their part, but that's no excuse to act in a shady manner.