Understanding Notifications & their states.
Nagios triggers notifications whenever a service check fails after no. of re_check attempts are completed and still the state of service doesn’t change from DOWN to UP.
Then these notifications are triggered to associated contacts or contactgroups.
Once the notification is sent,again the same will be triggered after the specified_interval in case the state of service is unchanged from DOWN.
The notifications can be sent to - mail, sms, winpop message, Audio alerts etc.
Basically whatever can be done via command line can be enabled within notifications definition.