Second to the leg muscles, your back muscles are the largest muscle group in the body. While the Gluteus Maximus (the booty) gets the prize as the most superficial muscle group, the Latissimus Dorsi of the back are the widest muscle group. The name itself when translated actually means "wide back." Being a powerhouse muscle group of the body, there are few upper body exercises you can do that do not engage the back muscle to some extent. There are however some specific movements you can do that work the back more than other exercises, and the Close Grip Lat Pulldown is one of them.
The Lat Pull Down is one of the most popular back exercises you can do in the gym. Even if you're a novice when it comes to weight lifting or building muscle you probably know what to do when you see a Lat Pull Down machine. The equipment is pretty self explanatory: sit down, grab the bar, pull down. I think the simplicity of the movement is actually what contributes to the fact that it's one of the most common exercises done incorrectly.