On a server, Nagios either runs as a service or daemon. Plugins that reside on the same server are being run by the Nagios; basically, they contact the hosts or servers of your network or on the internet. We can check the status by web interface; even notifications can also be received by email or SMS when something happens.
Nagios service runs certain scripts after a fixed time interval, so it acts as a scheduler. It can store the script result and run other scripts when it is changed.
