Bill, the "extra virgin" and the "virgin" refer to the quality of the olives used and the quality of the end product. Extra Virgin has less than 1% acidity and is made with the best olives (unbruised), virgin has slightly more acidity. What you should also be on the look out for is "Cold pressed" This means the oil is not heated and the natural anti oxidants and vitamines are not lost in the process. Basically, cold pressed extra virgin olive oil is the first pressing of the best olives where there has been no refining and/or heating.

Light olive oil is refined and has had much of the vitamines and antioxidants found in extra virgin olive oil removed. It is lighter and tastes 'better' because it is refined.

If you'd like a great read on oils and fats, I recommend Udo Erasmus' "Fats that heal Fats that Kill"

HTH
Nat