What is the purpose/mission of the electric power system?

What are the power system's main components?

What is a smart grid?

What are the expected benefits of a smart grid?

Does the whole power system operate at the same voltage level? 

Is the power system mostly operated with alternating current? 

What is the reactive power and how is it produced? 

Are there situations where direct current is preferred in the power system? 

Why are 3 (or even 4) wires used for power delivery?