Documentation
mod_auth_pam is a module for the Apache web-server. The Apache Software Foundation has a great primer on how to use Apache modules, which applies verbatim to this module. Please read it!
This module supports PAM authentication. There are many PAM modules. Most problems with mod_auth_pam can be traced to problems configuring the authentication modules themselves, so if you're using a non-standard one, please make sure that you know how it works!