This is the name of an exercise, which is also the name of the resistance machine it is performed on.
What muscles are exercised?
The exercise itself works the Latissimus Dorsi muscles (hence the name) which are located on either side of the middle to upper back, and if developed give the upper body a triangular shape. The Lat Pulldown also works the biceps, located on the front of the upper arms.
To perform the exercise
1. Select an appropriate weight.
2. Sit down on the seat and adjust the thigh pad so it rests on top of the thighs, not so it's crushing them.
3. Stand back up and take a shoulder width-and half again grip of the bar, this will usually be where it kinks.
4. Sit back down on the seat, placing thighs under the support again. Keep feet flat on the floor. Ankles, knees and hips inline with each other. Sit up straight, with arms extended.
5. To perform the exercise, bring the elbows down to the side of the body, bringing the bar down in front of the face so it touches the very upper chest. Make sure you maintain sitting up straight. You are trying to work your Lats, not your abdominals.
6. Return the bar to your start position, however, do not lock out the elbows.
Some people do this exercise bringing the bar down behind the head, this is fine, as long as it is performed correctly.
What tends to happen though is that the participant tilts their head forwards and flexes their upper body in an attempt to bring the bar down to their upper back. Therefore I believe that with bringing the bar down to the front of the body there is a lesser chance of taking the body out of alignment. Also, if you do your bog standard pull-up using your own body weight, not many of us bring our upper back up to the bar, rather our chest, right?