Resource File : It is used to store sensitive information like username, passwords with out making them available to the CGIs. Default path: /usr/local/nagios/etc/resource.cfg
Object DEFINITION Files: It is the location were you define all you want to monitor and how you want to monitor. It is used to define hosts, services, hostgroups, contacts, contact groups, COMMANDS, etc.. Default Path:/usr/local/nagios/etc/objects/
CGI CONFIGURATION File : The CGI configuration file contains a number of directives that affect the operation of the CGIs. It ALSO contains a reference the main configuration file, so the CGIs know how you’ve configured Nagios and where your object DEFINITIONS are stored. Default Path: /usr/local/nagios/sbin/