Generally when a bodybuilder says he does "4 sets of 6-12 reps" he means that he does 3 progressively heavier warmup sets (not to failure), building up to his heaviest weight for 1 set of 6-12 repetitions to failure.
The idea being that if you can't get at least 6 reps with the weight, it's too heavy... and if you can get 12 or more reps, it's much too light. So he will either add small amounts of weight often and stay in that range, or wait till he can hit 12 reps with a certain poundage then add a great deal of weight and start over again next time down at 6 reps (building that up to 12, and so on, and so forth).
Doing 4 "work sets" to failure with the same exercise isn't very common in bodybuilding. Though some people train this way.
Yeah it's a newb question, but you also need to realize that it doesn't matter much. You need to find what works best for you, there is no 1 right answer.