A Notification Group is a group of Notification Recipients or other groups. When a notification message is sent to a group, it is sent to all members of the group (subject to the conditions defined for each member).
By using the schedule option available for each member, you can use a Notification Group to reflect an on-call duty schedule. For example, you may have certain people who handle server problems during the business day and others who are on-call for after-hours problems.
Jobs that you set up don't need to know about the details; they can be configured simply to notify a group. Within the group, you can specify which members should receive messages at which times.