Pluggable Authentication Modules is a suite that enable the local system administrator to choose how applications authenticate users.
It's used on both Linux and BSD based systems (it's on your computer too if you're using Mac OS, because it is FreeBSD derivative).
What you could be most interested in are:
- authentication rules in
- modules you can use listed in
/usr/lib/pam/(for Mac OS) or
- documentation of PAM and it's most popular modules.