Pam is a pluggable authentication module for Linux.
Linux traditionally relied upon password authentication based on passwords stored in the etc/passwords directory. However, many new authentication schemes have arisen, including smart cards and replacements for the etc/passwords file. PAM makes it easier to include authentication schemes as part of a Linux implementation.
PAM is included in SourceLabs self-support offering for Linux.
PAM documentation can be found here
A good overview article published in late 2007 can be found here