The Work Breakdown Structure (WBS) is a very important document for the success of any project.This document presents a complete list of tasks required for an entire project from start to finish. A WBS is a graphical hierarchical representation of the project at hand. It often serves as a tool for everyone on the project team to use frequently to check progress, create to-do lists, and more. With that said though, it is important to remember the Work Breakdown Structure is not simply a to-do list, and should be treated in higher regard as such.
Take a look at Ann Gordon's article, What is a Work Breakdown Structure? to find out more about what the document is.
This article, The Role of the WBS in the Project Lifecyle Explained will assist in understanding exactly where the work breakdown structure fits into the overall project, and will provide examples of both the project life cycle and the work break down structure.