As a personal trainer this is a question I get asked more than most even though it is a pretty striaght forward answer people never seem happy with the answer given. To put it simply high reps and medium weight are best for size and low reps high weight are best for strength. So what is considered a high rep I hear you ask?
High reps would be considered anything from 8+. I try not to go over the 15 rep range as it seems to long to get the burn and unneccesary. So anywhere from 8 – 15 reps is ideal for size. Now what weight should you be doing for those reps…well whatever weight you can do 8 – 15 reps for 3 sets if the correct weight but usually this is around 60% of your maximum lift of 1. So if your max bench is 100kg then your 8 rep range should be around 60kg or there abouts.
For low reps and strength I usually stick in the 3 rep range and work my way up to my maximum weight of 3 reps. This give me enough reps to stimulate muscle growth as well as strength. Now this is the basics of it but to be honest it's all you need to know and none of this will be that important if you have not got your diet in check. So go down and get yourself a George Foreman grill here and find yourself a good butcher and start eating plenty of lean meat!