Light or extra light in this case refers to the color and flavor of the olive oil, not its calorie content. Extra-light olive oil is pale and mild, as its been ultra-refined. As extra-light olive oil has a higher smoke point* than regular or extra-virgin olive oil, its best suited for use in baking or any kind of cooking where a neutral-tasting oil is needed.
For salad dressings or other dishes where the flavor and fruitiness of olive oil are important, opt for extra-virgin olive oil, which has the purest taste. The oil is pretty much fresh from the fruit. It may be filtered but no heat is used to refine the oil. And because of its strong flavor, a little olive oil really does go a long way.
*The smoke point refers to the temperature at which cooking oils start to break down.