A big project feels overwhelming as long as it's a single line in a charter. The Work Breakdown Structure (WBS) turns it into concrete, estimable, assignable tasks.
See also the detailed definition on Wikipedia (Work Breakdown Structure).
A Work Breakdown Structure (WBS) decomposes a project into progressively smaller pieces: phases, then deliverables, then work packages, down to tasks precise enough to be estimated and assigned to one person. It answers a simple question: "what is this project actually made of?"
Unlike a schedule (which answers "when"), the WBS only answers "what". It's the foundation on which estimates, scheduling and budget are built — in that order. The PMI makes it a core deliverable of planning in its PMBOK framework, and NASA did a lot to popularize the method on its space programs.
A single overarching goal — "E-commerce site redesign". The top of the tree, never assigned directly to a task.
The major stages of the project life cycle: design, development, content, testing, launch.
What must be produced within each phase: "Desktop mockups", "Product page", "Checkout flow".
The executable level: small enough to estimate in hours or days and assign to a single owner.
The 8/80 rule: a final-level task should represent between 8 and 80 hours of work. Below that, the breakdown gets too fine to manage; above it, it's still too vague to track precisely.
Estimating 40 small, precise tasks is more reliable than estimating one big vague block.
Systematic breakdown surfaces work packages that would have been underestimated by thinking "roughly".
Once the WBS is set, each task can be placed on a Gantt chart with its dependencies.
Each work package has an identified owner, avoiding "orphan" tasks nobody feels responsible for.
Build your task tree, link tasks together and visualize it all on a Gantt chart. Free.