Work breakdown structure

A WBS is a hierarchical description of work oriented towards the delivery of a project. Each descending level in the WBS represents an increasingly detailed description of the project deliverables.
All steps of a project are outlined in a work breakdown structure flowchart, making it an essential project management tool for planning. The first two levels of the WBS – the root node and Level 2 – define a set of planned results that collectively and exclusively represent 100% of the project scope. At each subsequent level, the children of a parent node represent 100% of the scope of its parent node. It is common to have three levels of decomposition in the WBS, although it is possible to have a fourth and even a fifth level in case of very complex projects.
In short, the WBS defines the what of the project.