Shadow (slackware uses this)
Your passwords are encrypted. Instead of storing the encrypted password in /etc/passwd which is world-readable, it is really "shadowed" in /etc/shadow which is not world readable. If someone can't read your encrypted password then they have nothing to throw a dictionary attack at.
(pluggable authentication module)
Primarily used in redhats and redhat-alikes.
PAM tries to be a catch-all authentication system for systems with many users and many network apps that require authentication. Applications can therefore "plug in" to PAM for authentication so each and every application does not need to run its own password database. (sort of like Microsoft's Single Sign On domain system)
The problem is when apps REQUIRE pam, slackware users are screwed, or have to become very clever...
Some apps require PAM by default, but accept a compile-time option such as --with-shadow or similar. Read the docs, or ./configure --help