Work Package (WP)
Natural subdivision of a Control Account (CA). A work package is simply a task/activity or grouping of work and is the point at which work is planned, progress is measured, and earned value is computed. It can be translated into different terms in different companies and functions. It can be a design job, a tool design package, a build-to-package, a shop order, a part number, a purchase order, or any other definable task/activity at whatever level of control is normal for program management within the company. Below are topics that address work packages.
Planning Packages (PP)
A logical aggregation of work, usually future effort that can be identified and budgeted, but which is not yet planned in detail at the work package level.
Integrated Master Schedule (IMS)
Contains all the detailed discrete work packages and planning packages (or lower level tasks/activities) necessary to support the events, accomplishments, and criteria of the IMP (if applicable). 
Program Critical Path
A sequence of discrete work packages and planning packages (or lower level tasks/activities) in the network that has the longest total duration through the contract or program that is calculated by the schedule software application. Discrete work packages and planning packages (or lower level tasks/activities) along the critical path have the least amount of float/slack (scheduling flexibility) and cannot be delayed without delaying the finish time of the entire work effort.
Budgeted Cost for Work Scheduled (BCWS)
The sum of the performance budgets for all work scheduled to be accomplished with a given time period. This includes detailed work packages, planning packages, AE, plus Level of Effort (LOE) packages. 
Budgeted Cost for Work Performed (BCWP or Earned Value)
The value of completed work expressed as the value of the performance budget assigned to that work. This is equal to the sum of the budgets for completed work packages, completed portions of open work packages, AE earned on the base accounts, and the value of LOE activities. 
AcqLinks and References:
- Defense Acquisition Guidebook (DAG) – Chapter 11
-  DoD Earned Value Management Implementation Guide (EVMIG)