Hi, im looking forward to define muscle.
The "skinny" on defining muscle is to lower bodyfat. If your bodyfat is above single digits. you won't have the razor-sharp definition that the "muscle definition" crowd is after. Once your bodyfat is low enough, it doesn't matter what training scheme you use - you'll be able to see your muscles.
Light weights will do nothing for you except waste your time. Muscle definition is largely a product of body fat percentage. If you want to be more defined, you need to lose fat. That's assuming that the muscle is there to define. To get more muscle, get strong. Lift heavy things.
Ooops! Doc beat me to the punch.