Here's my input on Mass, Strength and Toning. When you're working out, you will get a High, Medium and Low Return for each of these three results of training.
6-8 Reps with High Weight - High Mass Return, Medium Strength Return, Low Toning Return
- You will build muscle mass quickly, with some gains in your strength. (You couldn't NOT get any strength gains, as pushing around huge amounts of weight will build up your strength no matter what.)
8-10 Reps with High/Medium Weight - Medium Mass Return, High Strength Return, Low Toning Return
- The "High Weight" will not be as high as the 6-8 Reps, as you are supposed to train to failure, meaning you can't train past 8 Reps at the given weight. (If you can push out 2 or more Reps, it's time to increase the weight!) You will notice strength gains, and some muscle mass, and this is a good, overall workout.
10-12 Reps with Medium/Low Weight - Low Mass Return, Medium Strength Return, Medium Toning Return
- While you won't build mass quickly, you will be burning huge carbs and making significant strength gains, while also enjoying good toning of your muscles.
12-15 Reps with Low Weight - Low Mass Return, Low Strength Return, Medium Toning Return
- A great starting set for beginners, and for accomplished bodybuilders who are looking for massive toning. You won't build huge mass, or make significant strength gains, but you will attack your muscles and make them more pronounced and visible.
Now, the above is based upon the rule that when you train, you train until you simply cannot push the weight up any more on the last rep of your set. If you're doing 8-10 reps, and on the 10th rep you try to push out 11, and succeed, try it again. If you try and push out 12, and succeed, this may mean it's time to up your weight.
As you exercise, and stick to a strict regimen, you will notice significant changes in your strength. A good rule of thumb is to look at your workout once a week, or once every two weeks, and increase your weights. Even if you think you can't do it, you can surprise yourself significantly!
Good luck!