What is a Work Breakdown Structure?
The name says it all.

The Work Breakdown Structure (WBS) allows you to break down the work into manageable pieces and then assemble those pieces in a structure that reflects your management approach for the project. Not surprisingly, the corresponding behavior is task control.

Work Breakdown Structure (WBS): A deliverable-oriented hierarchical decomposition of the work to be executed by the project team to accomplish the project objectives and create the required deliverables. It organizes and defines the total scope of the project. Each descending level represents an increasingly detailed definition of the project work … from A Guide to the Project Management Body of Knowledge (PMBOK® Guide) Third Edition.