Nagios Core is perhaps best thought of less as a monitoring tool, and more as a monitoring framework. Its modular design can use any kind of program which returns appropriate values based on some kind of check, such as a check_command plugin for a host or service. This is where the concepts of commands and plugins come into play.

For Nagios Core, a plugin is any program that can be used to gather information about a host or service. To ensure that a host was responding to PING requests, we'd use a plugin, such as check_ping, which when run against a hostname or address—whether by Nagios Core or not—would return a status code to whatever called it, based on whether a response was received to the PING request within a certain period of time. This status code and any accompanying message is what Nagios Core uses to establish what state a host or service is in.

Plugins are generally just like any other program on a Unix-like system; they can be run from the command line, are subject...