You're worried that if you start lifting weights, that you'll become muscle-bound, and then one day, if you stop it will all "turn to fat".
Uh-huh. And you probably still believe in the tooth fairy and worry that an asteroid might kill you... We can't help you if you're going to be irrational about this. Physiologists will tell you that it is virtually impossible for the vast majority of both men and women to get "big" from weightlifting unless they go to extremes and also take additives like steroids.
Of course, when you meet people who look too big, and ask them if they took steroids, what do you think the answer will be? Of course, they will deny it; because society frowns upon it, it is unhealthy, dangerous, stupid, in some cases it is illegal, and if nothing else, it is seen as "cheating". That woman you've seen with the bulging biceps? Steroids. The guy who is 5'5" and 4 feet wide? Steroids. The guy with arms as big as your legs, thinning hair, a bit of acne? Steroids.
If you lift weights without steroids, you won't get big; you'll slowly add a small amount of muscle, while losing fat (unless you continue to eat like a pig) up to your own body's natural potential.
And that's what you want! The primary goal of normal weight training is to affect your skeletal muscles directly to change your physical appearance, body composition, strength and basal metabolism. You MUST apply as much weight against your muscles as they can stand in a set of 10 - 15 repetitions (without acute, sharp pain) in order to cause them to firm up and change. That's the fact.
Weightlifting, which is a principally anaerobic (meaning NOT-aerobic) activity, works in an almost opposite fashion from aerobics. The focus is on increasing the resistance (weight) NOT the repetitions. This forces the muscles to change (less focus on the heart than aerobics; more on the skeletal muscles), but there still is a cardiovascular benefit.
In other words; you can't change the muscles with light weights, and if you don't change the muscles, you won't drive up your metabolism. If you don't drive up your metabolism, you won't lose weight and keep it off? Why? Because the increased metabolism burns off more calories every day, even on days when you don't work out. Aerobic activities only burn off the calories while you are working out and for a much shorter period after the workout than with weight training.
Again: You will NOT get big - that almost always takes steroids.
There really is no such thing as "toning" being different from any other anaerobic form of exercise. All resistance training, in order to have an affect on a muscle MUST apply force against the muscle to cause it to change. There is only one form of change. Aerobic activity changes muscles in the same way as weightlifting - but by applying so little resistance, the change and rate of change is also miniscule. In other words, you can do aerobics classes to the cows come home, but it will not cause a fraction as much loss of fat, nor firming up as will weightlifting - Aerobics is designed and intended to primarily affect the cardio-vascular system (heart, lungs and circulatory system NOT the muscles or physical appearance!!!!!!
In ANY form of exercise, when the muscle is challenged, the muscle fibers and fiber bundles hypertrophy (grow and replicate) while also becoming neuromuscularly trained to respond to commands more effectively.
Weightlifting is simply much more effective than aerobics (including running, steppers, biking, etc) at this.
If you a skinny (less than 12% body fat) woman who hates exercise and has scrawny arms... of course, you'll gain some volume in the form of muscle - in fact, that's the point! You'll look better, improve bone density, have more everyday strength and a high metabolism. Overall, you'll fill out slightly, and become more firm. On the other hand, if you really want to look like Ally McBulemia (Calestra Flockhart) then please stay the heck away from me - that's just plain gross!
If you are an attractive woman who rarely works out other than some running or aerobics classes (meaning you're 20 - 24% fat) you'll probably stay the same weight because you'll lose fat and gain a smaller volume of muscle - overall you'll lose inches and firm up.
If you are overweight and trying to lose 15 or more pounds, a combination of weights and cutting back on soft drinks, sugars, breads and alcohol will cause you to lose inches and firm up - there would be no muscle noticeable - just a leaner, firmer you.
If you are a broad-shouldered, thick-boned, thick-wristed woman with a higher testosterone level, like an Olympic swimmer; yes, you could get bigger than most women desire, by lifting. But women like this are actually fairly rare.
Remember: You MAY gain weight but will LOSE volume (unless you are already anorexic!)
Muscle is more dense than fat.
Gains in muscle increase basal metabolism, which strips off fat.
Again: You will NOT get
big - that almost always takes steroids.