When I was younger, I decided to start lifting weights. I made good progress in my two years of casual lifting (reaching a bench press of 300 pounds), but looking back, I realize I could have made much better progress had I actually known what I was doing.
I did all of my workouts at home, so I never really had a trainer to help me know what to do. If there was one thing I would say contributed to my lack of real progress, it was the fact that I never knew how many reps to do.
Studying for the National Federation of Professional Trainers' certification exam, I discovered that it's really quite simple:
To build muscle, strength, and size, do 4 to 6 reps
To improve general fitness and stamina without packing on too much muscle, do 12 to 15 reps
To build endurance, tone muscles, and to burn fat, do 20 to 25 reps
Why does it matter how many reps you do?
For Size and Strength: 4 to 6 Repetitions
When you do low rep exercises with lots of weight, you use the myofibrils in your muscles - particularly the white fast twitch muscle fibers. With 4 to 6 reps, you use up all of the energy in these fibers, but you don't do serious damage to the tissue when you reach muscle failure. If your muscles fail after 1 to 3 repetitions, you risk permanent tears in your muscle.
For Fitness and Stamina: 12 to 15 Repetitions
With sets of 12 to 15 reps, your muscles run out of short-term energy - the energy stored in your muscles. All of your red fast twitch muscle fibers are stressed, and thus they expand when you recover. If you were to do sets of 7 to 11 reps (the mistake I made), you wouldn't work either your red or white fast twitch muscle fibers optimally - leading to limited growth and strength increases.
For Endurance and Toning: 20 to 25 Repetitions
When you do high rep, low weight exercises, you use up all the energy in your muscles and force your body to bring more (which is why you burn more glucose, meaning less stored as fat). You use all of the power in your red slow twitch muscle fibers, causing them to grow and store more energy in the future. These muscle fibers give you the endurance that you need to contract your muscles for a long time, hence the increase in your endurance. With sets of 16 to 19 reps, you are using both red fast and red slow twitch muscle fibers - meaning neither get stressed optimally.
What's your goal? Do the right number of reps to reach it faster!