Work Breakdown Structures
The following notes are copied from my
Project management quick reference
Work Breakdown Structure Technique
Term Description
Step 1:Decide upon the product to be delivered from this project. The key inputs into this process are the Scope Statement and the Scope Management Plan.
Step 2:Identify the major deliverables. Deliverables are both the subsystems of the product and also the process and artifacts required to run the project.
Step 3:Decompose the deliverables into work breakdown packages where the scope of each package is optimal for managing the effort. A package should be small enough to manage and large enough to appear meaningful when reporting.
Step 4:Review the WBS with the appropriate stakeholders and update as required to ensure the WBS is sufficiently detailed to define the scope while high level enough to be manageable.
ActivityThe scope of a work package is further decomposed during the Activity Definition process into activities. An activity is a grouping of related tasks.
Code of AccountsA Code of Accounts is the labeling scheme used to link the work packages together and to facilitate effective communication. The PMBOK section number scheme is an example of a Code of Accounts.
TaskA Task is the smallest subdivision of work in the project schedule. Tasks are grouped into activities and activities are grouped into work packages.
WBS ElementEvery component in the WBS is a WBS element.
Work Breakdown PackageThe WBS Package is the lowest level of the WBS. Each package should be decomposed to a manageable scope while remaining large enough to be meaningful when reporting to the stakeholders.
Work Breakdown StructureA WBS is a hierarchical representation of the elements of work required to compete the project. The WBS can be organized around the subsystems in the product, the project team organizational structure, geography or any other logical grouping. Always include the work of project management in the WBS.

sample Work Breakdown Structure diagram
The following notes are copied from my
Project Management Office Quick Reference
The key definitions for these tables come from PMI guidelines for portfolio management. In brief:
  • Portfolio -- a business focused grouping of programs.
  • Program -- a grouping of related projects worked for a common purpose.
  • Project -- a temporary endeavor to create a unique product or service.
  • Package -- the lowest level component in a work breakdown structure.
  • Activity -- one step lower in the decomposition of a package.
  • Tasks -- the work assignments.
Schematic for a project repository

Typically a project schedule shows tasks as the rows in the schedule. Activities are groupings of tasks and packages are the major headings within the project schedule. There are times when it is appropriate to track work at the task level. However, it is often just as effective to focus on the activity instead.
The following notes are copied from my
Six Sigma Quick Reference

A work breakdown structure (WBS) is both a product management and a project management tool. As a product management tools, a documents the product to be developed, documents the product that is now supported and continues to document the product as it was in the past. A WBS should therefore be developed and maintained both as a tool for looking ahead and as a source of information for what previously was.

The project management use of a WBS is primarily focused on defining the major deliverables from the project so that those deliverables can be assigned to appropriate resources. The WBS then serves as a tool to clarify the project management approach and it should be archived to serve as a starting place for the next project. A very simple WBS is shown to the right. 		A work breakdown structure diagram

Note that there are predefined WBS templates available through the Project Management Institute, the USA government and other bodies for many products and projects. In some cases use of those templates is mandated by law or by contract.