JIRA is an event-driven system. This means that usually when an action occurs (for example, when an issue is created), JIRA fires off a corresponding event. This event is then picked up by components that are designed to listen to the event. Not surprisingly, they are called listeners. When a listener picks up an event, it will perform its duty such as keeping issues up-to-date with changes or sending an e-mail to users watching the issue.

This mechanism allows JIRA to process operations asynchronously. The advantage of this model is operations, such as sending e-mails, and it's separated from JIRA's core functions like issue creation. If there is a problem with the mail server, for example, you will not want this problem to prevent your users from creating issues.

There are two types of events in JIRA: