What Is The Difference Between Authentication And Authorization?

Authentication

Authentication verifies who you are. For example, you can login into your Unix server using the ssh client, or access your email server using the POP3 and SMTP client. Usually, PAM (Pluggable Authentication Modules) are used as low-level authentication schemes into a high-level application programming interface (API), which allows programs that rely on authentication to be written independently of the underlying authentication scheme.

Authorization

Authorization verifies what you are authorized to do. For example, you are allowed to login into your Unix server via ssh client, but you are not authorized to browser /data2 or any other file system. Authorization occurs after successful authentication. Authorization can be controlled at file system level or using various application level configuration options such as chroot(2).
Usually, the connection attempt must be both authenticated and authorized by the system. You can easily find out why connection attempts are either accepted or denied with the help of these two factors.