A Work Breakdown Structure (WBS) is used to structure ideas, hypothesis and objectives. Work packages belonging together will be grouped in one hierarchical path in logical and chronological order. Each descending level of the WBS represents an increasing level of detailed definition of the project work. The WBS helps you to detail cost estimations and is a guide for schedule development and control. Nevertheless, the WBS is flexible and can be revisited and updated.
For some hints how to design a good WBS click here.