The beauty of rowing is that it makes you use a much larger range of movements with your body than some other exercises.
You will stretch muscles and rotate joints as the seat moves back on your row machine in order to carry out the task. In the process you will find that you tone muscles in your arms, legs, back and stomach simultaneously.
Obviously the legs and arm muscles are used during the stroke movement but both the back and abs are called into play as well. As a result of this, rowing is an exercise that is extremely good for all-round fitness and it works most of the muscles that people are looking to improve in tone. In particular, your abs and butt will both see marked improvement over time.