Critical Path
The critical path is the series of tasks that dictate the start date and end date of a group of linked tasks. Any task on the critical path is a critical task. If progress on a critical task slips, so does the progress of that group of tasks as a whole.
To quickly identify which tasks in a linked group of tasks are on the critical path, right click on a task (in the Tasks List or the Gantt Chart) and select the Select | Critical Path menu option.
A non-critical group of linked tasks may become the critical path, if they slip enough to effect the schedule end date.