Earned Value, also known by the 2 letter anagram EV, refers to the precise and specific value of all sum total of completed project work to date. These earned values are expressed in terms of a budget that has been formulated previously, has been reviewed, and has been approved, and has in turn been assigned to this particular work element by a particular work breakdown structure component or in some cases by a specific schedule activity. The term earned value can also be referred to by the alternative term of budgeted cost of work performed, which in turn is also often referred to by the 4 letter anagram BCWP. Keeping a careful tab on the earned values within the particular components of a project is essential for those who are maintaining the budget of a particular project, or else the numbers may not accurately represent the reality of the situation.
This term is defined in the 3rd and the 4th edition of the PMBOK.
Examples will help understand these concepts better. also, when do I calcuilate EV? After the project is complete and the rewards of the project objective are being realized? or during the course of the project when no results are out there to measure profit?