 <p>PAM - Pluggable Authentication Modules - is a general mechanism which allows you to configure the authentication mechanism used by programs on your system, without requiring recompilation of the relevant programs. So, for example, you can change your system's basic login to use MD5 hashed passwords instead of DES encrypted passwords, without changing (recompiling) the login program.</p> <p>RADIUS (Remote Authentication Dial-In User Service - but there's a suspicion it is a backronym) is a specific authentication mechanism. There are PAM modules that can authenticate via RADIUS, just as there are other PAM modules that can authenticate with LDAP or Unix password files or ...</p>