A work breakdown structure is a hierarchy that divides a project into smaller deliverables and work packages until each piece is small enough to plan and assign.
The top level is the whole project; each level below breaks it into parts. The lowest level, work packages, can be estimated, scheduled and given an owner.
Example
Website launch: Design (wireframes, visual design), Build (frontend, backend, content), Launch (testing, marketing, go live).
A WBS in Starlex
Epics, issues and sub-tasks form a three level breakdown, and a diagram can draw it as a tree.