Let us define a few of the terms that Zenoss uses, so that we can more accurately understand the concepts.

From ticket 6407

Event - Something that happens. A hard drive is full, a ping stops responding, etc. You can also get clear events, which means that the bad event will get cancelled and sent to history

Alert - Totally separate from events. An action that occurs when a user-defined where clause is met. Usually when a certain event comes in. Think of this as the email that goes out. By default, there are no alerts on events. Events pour in, and you get no emails, until you setup alerts.