It is the work defined at the lowest level of the Work Breakdown Structure (WBS) for which cost and duration can be estimated and managed.
Work packages are defined in the “Create WBS” process of Scope Management.
Decomposition is the process through which the total project work is divided into work packages. The size or level of detail of work packages varies based on the size and complexity of the project.
It is a distinct, scheduled portion of work performed during the course of a project. What many people refer to as a “task” in the real world is often called an “Activity” in the PMI Terminology. Activities should be of enough size to estimate, Schedule, monitor, and control.
Work packages that are created in the “Create WBS” process are decomposed in the “Define Activities” process, to form activities that are required to produce the work package deliverables, thus achieving project objectives.
So, in summary, WBS is the decomposition of the entire scope of work. Its lowest level is a “Work package”. A Work package is in turn decomposed to form “Activities”.
Work Package vs Activity Comparison