A work package is is a group of activities for which work is estimated, scheduled, monitored, and controlled. A work package defines work at the lowest level for which cost and duration can be estimated and managed. It is essentially a sub-project.
One should only decompose projects down to the work package level at which the additional detail assists in the planning and management of a project. Greater levels of detail provide no incremental benefit to the project manager.
A good way to measure the progress of a project is to set up many short-duration work packages. When each work package is clearly defined in this manner, one can easily determine whether it has started and finished on time. Conversely, if a work package is several months long, it is quite difficult to tell if the work package is really on track. In the latter case, a likely outcome is that the manager will only discover that there is a problem when the work package approaches its scheduled finish date.