To be able to actually execute the project, the project scope is broken down into manageable tasks by creating a work breakdown structure (WBS). In other words, a WBS is a deliverable-oriented hierarchy of the work that must be performed to accomplish the objectives of and create the deliverables for the project.
Decomposition is a technique used for subdividing the project deliverables into smaller, manageable tasks called work packages. The WBS is a hierarchical structure with work packages at the lowest level of each branch.