Isotonic exercise is the most commonly prescribed activity in most Gyms.
Quite simply, isotonic exercise is working muscles against resistance. So whether you use hand weights (dumbells) or are using a piece of gym equipment, if you are resisting the muscles being exercised then you are performing isotonic exercise. The more resistance you are using when exercising, the more muscle fibres are being used and the more effect it will have.
Isotonic exercise is designed to strengthen and tone the muscles being used so providing it is performed regularly you can expect to see flabby muscles toned and inch loss.
It is crucial that the right amount of resistance is used of course. Neither excessive strain nor too little resistance and that is where your Instructor comes in. How much resistance she will ask you to use on each exercise and how many repetitions she will ask you to do will determine how effective the exercise will be and as the weeks go by, she will adapt her advice according to how well you are doing.