What is Work Breakdown Structure (WBD) and how does it affect the work estimates of tasks/activities?

Work breakdown structure defines the work activities required for the project and the sub activities of each of the work requirement. The breakdown goes down to levels where all the work required is clearly understood. Work need not be broken down further than that. Work breakdown dictionary includes additional details that help define the tasks. Time and effort estimates can be accurate when everything about the work and dependencies are known.