A scheduled alert is an alert that runs on a regular interval, making it a type of scheduled search. You can specify the schedule and trigger conditions, and you can define the alert actions that take place when the alert is triggered. For example, you can monitor sales using a scheduled alert that searches daily for purchase events. The alert can trigger an alert action when there are fewer than 500 purchases.
Scheduled alerts are similar to scheduled reports, which also run on regular intervals. The difference is that scheduled reports initialize one or more alert actions each time they run, while scheduled alerts only initialize alert actions when they are triggered.
In the Alerting Manual: