Work Breakdown Structure (WBS) is a hierarchical decomposition of a project or other entity into phases, deliverables and work packages.
It is a tree structure, which shows a subdivision of effort required to achieve an objective, such as a program, project, and contract. The WBS is developed by starting with the end objective and successively subdividing it into manageable components in terms of size, duration, and responsibility (e.g., systems, subsystems, components, tasks, subtasks, and work packages) which include all steps necessary to achieve the objective.
The WBS is stored as a metadata field (ex: 1, 1.1, 1.1.1, etc.) and can be viewed graphically in a Gantt Chart.