Guidelines to Designing Your Own: "In Practical Programming, author C.M. Rippetoe uses a chart to explain which rep ranges accomplish which things in the intermediate and above stages. The 1–3 rep range is best for pure strength, myofibrillar hypertrophy, ATP storage efficiency, neural adaptation, and bone density. The 3–5 rep range is best for power and explosiveness, and 20 reps are best for sarcoplasmic hypertrophy. Extremely high rep ranges are best for pain tolerance and lactate production. Catch all that?
Don’t worry about it. Basically, 1–5 reps are best for pure strength (I wouldn’t recommend anything lower than three reps for anything lower than the intermediate level because of neurological demands). Anywhere from 6–15 reps are best for hypertrophy (highly individual), and really any rep range works for endurance depending on how much endurance is required. Anything over 20 reps is probably a little much. However, people make the mistake and think that 12 reps won’t do anything other than hypertrophy. Wrong! Twelve rep maxes will make you good at things that require the endurance of something that replicates a twelve rep max."