Rowing is one of the best cardio exercises there is as well as exercising 84% of the muscles in your body.
The advantage of a rowing machine is that it will work your entire body. A treadmill will focus on the lower body but a treadmill is better for burning the maximum number of calories per hour. Both can be used for low-impact exercise.
Rowing provides upper body conditioning and gives the upper and lower back and the shoulder muscles an excellent workout. As the seat also slides, rowing also provides a lower body workout as well.
Every stroke uses the calf muscles, quadriceps, hamstrings, glutei, abdominals, pectorals(chest), biceps, triceps, deltoids( all arm muscles)and upper back.
The hands and wrists become stronger as well due to gripping the rower handles.
To get the best results it is important to use the right technique. So either ask a trainer to watch your technique or look it up on youtube.