Similar to what Pa�ul is saying, in order to attain any fitness goal, the weight being used must bring you to almost failure by the last rep. Regardless of whether or not you are doing 20 reps or 5 reps, in order to see results you must use a heavy enough weight to bring your muscle to momentary failure (but obviously avoid getting there).
If you do 15 reps with "x" weight but could really do 20-25 reps, you are simply lifting for fun, not for results. Our body needs to be pushed in order to change position, shape, and size.
High weight low reps may work great, but not always. As with anything you do in life, you must adapt to changes. As your strength progresses and you are building muscle you will notice that the same formula just isn't working for you, so try switching from high-weight low-reps to a higher number of reps and a slightly lower weight than when you first started with at the very beginning. Or vise versa.