What is Work Breakdown Structure (WBS)?
The WBS is a hierarchical decomposition of the total project scope into smaller, manageable pieces — everything the project will deliver, broken down until each element can be estimated, scheduled, and owned. Its golden rule is the 100% rule: the WBS contains all the work, and only the work.
If something isn't in the WBS, it isn't in the project. That single sentence resolves a surprising number of exam scenarios about "extra" work.
Worked example
"Build a customer portal" decomposes into design, development, testing, deployment — and development decomposes into authentication, dashboard, payments. Keep going until each piece is estimable and assignable. That tree is the WBS.