The heart of Nagios is the service definitions file
For host status monitoring
Using nagios-statd-server/client is easier
Using nagios-nrpe remote plugin execution is a bit more work, but can do things nagios-statd-server can't, especially for network monitoring
For network status monitoring
network services are tested from the host/server, unless you use nagios-nrpe
See the files in /etc/nagios-plugins for available commands