Work Breakdown Structure - WBS is a tool used to divide project work into smaller more manageable segments.
It can have as many as needed levels, however it is recommended to make them few (for example 3 to 4 levels) so that they can easily analyzed. Each node in the bottom level of a WBS is called a Work Package. The level above the Work Package is called a Control Account. The 100% rule in WBS states that every thing need be included in the WBS and nothing extra.
The following figure depict an example WBS from the book "A Practical Approach to PMP Prep", by Riad Thalji, where the fictional project manager created for a project to distribute laptops on as set of secondary schools in that state:
Trainer Riad Thalji