below explanation for this answer:
- Nagios runs on a server, usually as a daemon or service.
- Nagios periodically runs plugins residing on the same server, they contact hosts or servers on your network or on the internet.
- One can view the status information using the web interface.
- You can also receive email or SMS notifications if something happens.
- The Nagios daemon behaves like a scheduler that runs certain scripts at certain moments.
