Wikipedia defines the work breakdown structure, or WBS as it is known, like this:
A work breakdown structure WBS in project management and systems engineering is a tool used to define and group a project's discrete work elements in a way that helps organize and define the total work scope of the project.
The WBS provides the project manager and team with the necessary framework of tasks going forward to create detailed cost estimates and also to provide a major input to project task scheduling at the most detailed and accurate level possible. By going through the WBS motions, the project manager and team will have a pretty good idea whether or not they’ve captured all the necessary tasks, based on the project requirements, that are going to need to happen to get the job done.
Four key benefits to developing a WBS are:
WBS forces the team to create detailed steps
The WBS forces the project manager, team members, and customers to delineate the steps required to build and deliver the product or service. The exercise alone encourages a dialogue that will help clarify ambiguities, bring out assumptions, narrow the scope of the project, and raise critical issues early on.
WBS lays the groundwork for schedule and budget
It lays the groundwork for developing an effective schedule and good budget plans. A well-defined WBS enables resources to be allocated to specific tasks, helps in generating a meaningful schedule, and makes calculating a reliable budget easier.
WBS creates accountability
The level of detail in a WBS makes it easier to hold people accountable for completing their tasks. With a defined WBS, people cannot hide under the “cover of broadness.” A well-defined task can be assigned to a specific individual, who is then responsible for its completion.
WBS creation breeds commitment
The process of developing and completing a WBS breeds excitement and commitment. Although the project manager will often develop the high-level WBS, he will seek the participation of his core team to flesh out the extreme detail of the WBS. This participation will spark involvement in the project.
The downsides (sort of)
Of course, developing a WBS is not easy. It can be a painstaking process. And it can take quite a bit of time. A large WBS (one that identifies several thousand activities) can take many, many hours to develop. For another, it requires effort. There is a knowledge transfer and exercise of brainpower. The larger the scope of the project, the larger the WBS will be. More people must provide input and then approve the portion they are responsible to perform. Finally, the WBS requires continual refinement. The first iteration is rarely right and as the project changes, so does the WBS.