Characteristics of task groups
A task group determines most of its characteristics from the tasks it contains, rather than having characteristics of its own.
Name: Of course, a task group has its own name.
Type: A task group can contain 
Tasks, 
Milestones, and other 
Groups, but its own type is always Group.
Tasks, Milestones, and other Groups, but its own type is always Group. Effort: The effort of a group is the sum of the effort of all tasks it contains.
Duration: The duration of a group is the amount of time between the beginning of its first task and the end of its last task, not the sum of the durations of all its tasks. If a group contains three one-hour tasks all happening at the same time, the group has a duration of one hour, not three hours.
Scheduling: A group can have a specific start date, which can then affect the start dates of tasks inside it.
Reserve: A group can have reserve time, which is extra time allowed in case the tasks inside end up taking longer than they are scheduled to take.
Dependencies: A group can have dependencies just like a regular task.
Resource Assignments: If you assign a resource to a task group, the resource gets assigned to all tasks in the group. The group itself can't have any resources assigned.