The major difference between Active and Passive checks is that Active checks are initiated and performed by Nagios, while passive checks are performed by external applications.
If your interviewer is looking unconvinced with the above explanation then I will suggest you to also mention some key features of both Active and Passive checks:
Passive checks are useful for monitoring services that are:
Asynchronous in nature and cannot be monitored effectively by polling their status on a regularly scheduled basis.
Located behind a firewall and cannot be checked actively from the monitoring host.
The main features of Actives checks are as follows:
Active checks are initiated by the Nagios process.
Active checks are run on a regularly scheduled basis.
