Traditional approach is to choose the number of sets and reps to achieve intensity and volume that are best suited for your goal.
The major goals are Strength, Power, Hypertrophy (Size) and Endurance. Strength is achieved with the highest intensity, and endurance is achieved with the highest volume. Intensity is proportional to the weight lifted. Volume is proportional to the number of sets and reps. If you want to increase intensity, you need to decrease volume.
Strength/Power - do 4-8 sets of 1-5 reps with extremely heavy weights. To emphasize "power", do your lifts fast, explosively. To emphasize "strength" and muscle size, do them smoothly at a slower pace.
If you work out regularly for less than 6 months, do not select "Strength/Power" - your muscles are not ready for the extreme intensity of these workouts. The more experienced you are, the fewer reps (with heavier weights) you can do. The extreme case will be 8-10 sets of 1 rep, done with the weight that is very close to your One-Rep Max.
Size - do 2-4 sets of 5-10 reps each. This is the most traditional workout style, practiced by millions since 1970s. It improves muscle size and strength; however you will not reach your maximum strength.
You can also select "Size" if your goal is to loose some weight (body fat). The purpose of lifting weights for weight loss is not to burn calories during the workout, but to boost your metabolism. You cannot burn calories effectivly simply by lifting weights. Besides, those calories come from sugar that is stored in your muscles (glycogen), not from burning your fat. But boosted metabolism will burn your calories 24 hours a day. Calorie-reduced diet and cardio exercises reduce your strength, so you cannot do "Strength/Power" workouts during weight loss - choose "Size" and do 3-4 sets of 6-12 reps.
Toning is a vague term that often stands for "Low Intensity/High Volume". The same kind of training is done for Endurance. You may want to perform 2-8 sets of 15-50 reps with weights, or as many body weight exercises as you can (for example sit-ups, push-ups, chin-ups).