And there you go. This really is the best explanation of repetitions that I could find and sums it up better than I can explain. For information directly related to this, read the next section on "Muscular Hypertrophy".
Muscular Hypertrophy
Not all hypertrophy is created equal, and no, I'm not talking about building "lean" muscle and building "bulky" muscle. A more apt (but still terribly simplified) comparison is "functional/dense" vs. "superficial/bloated".
Sarcoplasmic Hypertrophy-This is an increase in the sarcoplasm in the muscle. Sarcoplasm is NON-CONTRACTILE TISSUE, but accounts for roughly 30% of the size of the muscle. In increasing sarcoplasm, you are actually DECREASING the muscle's contractile fiber density, resulting in less power/strength output across the muscle area. As you can see from the chart, from 10-12 reps and up is when sarcoplasmic hypertrophy comes into play. This is probably the type of "muscle-building" that bodybuilders get a rep for "all show and no go". However, it is not completely useless--simply being bigger in a chosen sport can have it's advantages and building size can help provide a better foundation for increasing your strength (the larger the cross-sectional area of a muscle, the greater it's potential for strength)
Myofibrillar Hypertrophy-This is an increase in the size of the actually muscle fibers, but NOT an increase in the amount of muscle fibers. The "density" of the muscle will actually increase. This sort of training is more "functional" and more important for athletes, as low reps build explosive power as well. Increases in size may not come as quickly as sarcoplasmic hypertrophy, but you still WILL get bigger if you train this way and eat properly. Low reps (1-5) are best for this.