"Notifications" usually refers to the event-related messages sent to e-mail addresses or cell phones.
Sending notifications is one among several rule actions that can be performed when a rule is triggered (See Rules). When you create a rule and add a Send To Notifier action, you will be able to select the notification group that will receive the message. For more information on rule actions, see Managing Rule Actions.
The key entities in the notification structure are Notification Groups, Escalation Levels, and Destinations.