Strength training is performed to increase muscular strength, endurance or both.
Muscular strength is defined as how much weight you can lift and muscular endurance is the ability to continue lifting repeatedly without fatiguing. Lower reps (less than 6) favour increases in muscular strength, whereas higher reps (more than 15) favour increases in muscular endurance. Generally speaking, the range of 6 to 15 gives the best balance of muscular strength and endurance. As improvements are best when fatigue is achieved upon completion, the 8 to 12 reps range has been adopted as a safe way for people to strength train.