Pluggable Authentication Modules (PAM) provides a system of libraries that handle the

authentication tasks of applications (services) on a Linux system. These libraries are loaded

dynamically and can be configured in the /etc/pam.d directory or in /etc/pam.conf. The PAM

modules are divided into four types:

auth (authentication): This module type is responsible for authenticating the user, setting up user

credentials, and initiating a session.

account: This module type manages account policies such as password expiration, access restrictions,

and checking user permissions.

password: This module type handles the updating of authentication tokens, such as passwords.

session: This module type manages tasks that need to be performed at the beginning and end of a

session, like mounting directories or logging.

Reference:

Linux PAM Documentation

Understanding PAM