A work breakdown structure is a tool that helps systematically identify the work necessary to achieve the project's goals. It is the first decomposition of what the project has to deliver into "chunks of work"—groups of activities that must get done. The WBS is a key step to be done before
trying to put together a schedule with detailed tasks and dependencies. In general, work breakdown structures show us where we are going and schedules show us how we will get there in time.
For larger projects, there is tremendous value in first decomposing the work into tangible deliverables and then further decomposing work into activities long before creating a schedule. The keyword here is manageable—decomposing large chunks of the work into areas that logically reflect how it will be executed and managed. Some WBSs are structured by functional groups or cross-functional sub-teams; some are broken down by development process phases. In large project organizations, the WBS chart can be aligned with organizational charts, functional groupings, or other areas of responsibility/authority.
For smaller projects, there are often questions about how detailed a WBS needs to be, or whether the separate step of creating a work breakdown structure need to happen at all. A graphical WBS can be an invaluable aid for visualizing and understanding the scope of work for even a modest project. But some project managers outline and decompose tasks directly into a scheduling tool and find this process satisfactory. In either case, what's important is whether a true, effective WBS has been created. Done right, a WBS's structure actually helps bring to light missing work, whether the project is small or large. The danger in going straight into an outline tool for a small project is that the mindset of a logical organization and decomposition of the work gets skipped, and the team ends of with a hodgepodge list of work to be done that may or may not be complete.
For a detailed introduction to how to create a good work breakdown structure, examples, and how to know you've gone to enough detail, see our guideline Planning and Scheduling: Task Identification and Work Breakdown.