A breakdown structure is a hierarchy of components of some aspect of a project, programme or portfolio.
Examples include: cost breakdown structure (CBS), organisational breakdown structure (OBS), product breakdown structure (PBS) and work breakdown structure (WBS).
For instance, if the root was the output of a project the decomposition could be the products that make up the output (PBS) or the work required to create it (WBS). In the case of the WBS, the bottom level of breakdown would represent the activities used in a network diagram.
If the root was the project or programme delivery team (OBS), then the components may show departments or skill groups at the higher levels and individuals at the lower levels.
A WBS and OBS can be used in conjunction to create a responsibility assignment matrix (RAM) that shows who is responsible for which activity.
With cost as the root, the breakdown structure could form the basis of cost accounting and reporting.
Some authors recommend the use of breakdown structures to assess and consolidate areas of risk in a risk breakdown structure (RBS).