An alert is an action that a saved search triggers on regular intervals set over a time range, based on the results of the search. When the alerts are triggered, various actions occur consequently.. For instance, sending an email when a search to the predefined list of people is triggered.
Three types of alerts:

Pre-result alerts : Most commonly used alert type and runs in real-time for an all- time span. These alerts are designed such that whenever a search returns a result, they are triggered.
Scheduled alerts : The second most common- scheduled results are set up to evaluate the results of a historical search result running over a set time range on a regular schedule. You can define a time range, schedule and the trigger condition to an alert.
Rolling-window alerts: These are the hybrid of pre-result and scheduled alerts. Similar to the former, these are based on real-time search but do not trigger each time the search returns a matching result . It examines all events in real-time mapping within the rolling window and triggers the time that specific condition by that event in the window is met, like the scheduled alert is triggered on a scheduled search.   _{Your Comment}