What is Earned Value in Project Management??

Earned Value at any point measures :
Whether a Work done is

1. Cost --> On Budget, Under Budget or Over Budget
2. Time -->On schedule , Ahead of Schedule or Behind Schedule
3. Actual Cost & Time Efficiency?
4. With Present Efficiency --> How much we shall be spending in future?
5. If to be within Budget --> What efficiency to be attained in Future?

Problem : GOOD CONCEPT, But at what level to be applied, WBS - Work Package, Summary Level or Activity level???