Lifting a light weight several times will definitely increase the endurance of your muscle, but it won't make you stronger and won't make you look leaner either. Lifting heavier weights, fewer times will increase your strength and increase your metabolism, eventually decreasing your body fat surrounding the muscle, which will make you look more lean and toned.
The reason many people are fearful of lifting heavier, is they don't want to "bulk up," bulking up happens when you are lifting a heavy volume of weights, eating high calories and have the right amount of hormones (more in males then females) or use supplements. Don't be afraid to challenge your body a little more with an increase in weights! Instead of doing 15-20 reps, try 10-12, just don't do more than 3 sets of 2 different exercises for a particular muscle group if you are afraid of "bulking up!"
Please view link below, great article on lifting with different weights!