I agree with the general theme within most of the answers already given. It varies and depends on both client conditioning and goals. I would generally start beginners with lower weight, higher reps. On the other hand, experienced clients will lift heavier weight, less reps with respect to whether they want to grow muscle, build strength, or increase power. The links provided in above answers are good to identify your basic guidelines of reps and rest. Apply those and then alter as needed based on client conditioning and progress (or lack of).