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.