The Notification Task allows you to send notification messages, and optionally capture and attach files. The Notification Task provides the same functionality as the Notification Action and File Capture Action. Those actions can only be run as Responses for a job or a step, but the Notification Task, because it is a task, can be the primary function that a job or step performs.
Generally, you would use a Notification or File Capture Action when you want to send a notification message in response to something that occurs during job execution (i.e., the notification is of secondary importance). You would use Notification Task when the notification is of primary importance.
For example, if you wanted to monitor a computer and send a notification message when it failed, you would create a job that uses a Computer Monitor Trigger and a Notification Task.