The truth is that this type of strength training doesn't burn more fat and the only way it will 'tone' your body is if you've created a calorie deficit that allows you to lose body fat. Using lighter weights for higher reps will help you increase muscular endurance and it does have a place in training routines, but that lean, defined look comes from losing body fat.
So, does that mean you shouldn't use the light weight/high rep approach with strength training? Not necessarily. How you lift weights depends on your goals and fitness level. But, for weight loss, it's great to use a variety rep and weight ranges. The general breakdown of reps and weight according to goals is:
- For strength gains: 1-6 reps, heavy weight
- For gaining muscle and size: 8-12 reps, medium-heavy weights
- For endurance: 12-16 reps (or more), light-medium weights
No matter what range you choose, you should always lift enough weight that you can ONLY complete the desired reps. If you're doing 12 bicep curls, choose a weight that allows you to 12 reps with good form. If you can do more than that, increase your weight.
Using all three ranges, whether you use them each week, each month or change them every few weeks, is a great way to challenge your body in different ways. For more check out Weight Training 101 and Best Weight Training Guide for New Trainers.