Any change performed on an JIRA issue creates a corresponding issue event. E.g. if you create an issue, Issue Created event is fired. JIRA mechanism in dependent on the issue events. They inform the system that something has happened and trigger appropriate actions within JIRA.
The two types of issue events in JIRA are:
System events that are fired by JIRA internally. They cannot be changed or deleted.
Tip: You can make system events inactive. - Custom events -