You will usually find a rowing machine near the exercise bikes and treadmills in a gym. It simulates the two handed rowing motion you would get from being in a row boat or rowing shell.
They are usually made up of a wheel with a chain around it which is then attached to a bar big enough to hold onto with two hands. Attached to all this is a long beam with a seat on it.
To use the machine sit on the seat with knees slightly bent and feet in designated area, grasp the cable attachment (small bar), and pull towards your torso, moving your shoulders backwards and pushing out with your chest. To return, fully extend arms with shoulders stretched forward and repeat.
Sometimes the seat is on rollers that can move it up and down the beam as you row. This means that you will not only feel the workout in your back, biceps, and posterior deltoids, but in your quadriceps too.
I find nearly everywhere I go these machines are left empty. Personally, I feel that they area great exercise for overall aerobic fitness, especially for the upper body. Coupled with a session on the bikes, treadmills, or stairmasters, they provide a more general and overall stimulation of your cardiovascular system than just running/walking/cycling alone.
The rowing machine is a great way to warm up before working out.