The work breakdown structure is often referred to as the WBS. From the PMBOK glossary, the WBS is "a hierarchical decomposition of the total scope of work to be carried out by the project team to accomplish the project objectives and create the required deliverables."
A WBS can be organized by activity or by deliverable. It is a way of breaking the entire project into smaller components. This makes it easier to assign the component activities and track the progress of the overall project.
The work breakdown structure is developed in the create WBS process. Some of the design principles for a WBS are
- 100% rule - the WBS captures all the deliverables of the project
- mutually exclusive elements - no overlap (between the breakdown at a given level; each level shows progressive elaboration)
- plan outcomes, not actions
- level of detail - generally the breakdown should continue until the lowest level is longer than one reporting period and less than 10 work days (The elements at this last, most detailed level are also known as work packages.)
- Wikipedia on work breakdown structure
- PMI article: Brotherton, S. A., Fried, R. T., & Norman, E. S. (2008). Applying the work breakdown structure to the project management lifecycle. Paper presented at PMI® Global Congress 2008—North America, Denver, CO. Newtown Square, PA: Project Management Institute.