When you start weight training/resistance training it is important to know what kind of repetition range is neccesary to achieve your goals. Whether your goal is powerlifting, increasing your strenght, Bodybuilding or muscular endurance then being armed with this knowlege will ensure you dont waste any time in making progress with your training goals.

For Powerlifting such as competition Deadlifting, Squating or Bench Pressing etc., lifting the most amount of weight possible for 1REP. This is know a 1RM (1 repitetion maximum) Power training invloves an initial warm up set of 10-12 reps then increasing the weight so that only 1-3 REPs are possible per set. This type of training will increase your power and some strenght but is not nessecarily the best to build muscle.

For Strenght training the reps are slightly higher, lifting a medium to heavy weight bringing the working muscle to failure between 6-8 REPS per set which will help strenghten and build muscle bulk. 

For Bodybuilding aslo known as Hypertrophy training you will be lifting medium weight bringing the working muscle to failure at 10-12 reps per set. These higher reps will help pump the muscle with blood and create expansion and growth of the muscle but not much strenght or power.

For muscular Endurance which is usually used to increase toning and sports performance the weight is low but the reps are high from 15-20 REPS per set. This type of training is usually used in general fitness regimes or in sports drills or circuit/interval training.

I believe for general training and increasing muscle mass its important to use the full spectrum of Rep Ranges to make sure you get the maximum benefit from all. 
 

Pyramid Training allows one to use the full rep range, for instance, 4 SETS of 12, 10, 8, 4-6 REPS. Weight increases as reps go down per set, working up to a final maximum-effort set of 4-6 reps. Then moving onto the next exericse and repeating that sequence again.

-APG.