nagios by itself is actually quite small. Nagios is use of plug-ins provides the breadth of its functionality. A plug-in is an external application that nagios calls to perform a specific task, such as checking the status of a host or service. Because there's a plethora of hardware and software out there, Nagios cannot possibly be written to support each type, so it relies on plug-ins to provide that support. Nagios has a standard plug-in library. which is maintained by the Nagios Plug-in Project.