If you are planning to buy a rowing machine, you cannot just go out and purchase one if you have no idea about them, we would suggest that you should read more about it on the internet, talk to your buddies who are into working out or go to your nearby gym and ask the instructor there about the dynamics and benefits of the machine so that you are fully informed on the subject.
Full Upper Body Workout
It is true that throughout the entire rowing exercise, you work your upper body or rather the machine makes you work your upper body. The main areas of focus are the back, shoulders and arms. When you are rowing with the machine, the rhomboids in your shoulders are constantly engaged as you are using these muscles to row. The back of the rower is continuously involved as it takes all the pressure and that is where the strength comes from, the trapezius and latissimus dorsi muslces in the upper and lower back are working throughout the exercise. The triceps and biceps in the arms and wrists are also involved actively in rowing and it is true that if you do not use them, you would not be able to row.
Lower Body Workout
Many people who use rowing machines use it solely for the purpose of working on their lower bodies because using the rowing machine has a lot of effect in those regions of the body. The muscles in the thighs (quads) gets worked on a lot when you use a rowing machine and you would see an amazing change in your glutes and calves and no one would ever be able to say that you missed leg day.