The pull up is sometimes referred as "the king of upper body movements" and for a good reason. This is a compound exercise that works every muscle on you upper body. It primarily targets the lats and biceps, and it also works the rhomboids, traps, chest, shoulders, triceps and forearms. Your core, abs and lower back will also assist as stabilizers.
Since pull ups work so many muscles at once, they will help you build much more muscle than by doing many smaller movements combined, this is especially great for people who don't have a lot of time to exercise. The pull ups will build muscle mass in your back and make it wider, so if you want that V-shape, you need to do pull ups. It will also improve your posture, making you look much better. This is especially important if you have a desk job. Just a few pull ups a day can help you correct back problems caused by sitting slouched for extended time.
Pull ups are a great test of strength and fitness. But they are also very hard to perform. If you have never done them before, most likely you won't be able to do even one. Don't get discouraged though, the pull up is very important to learn because it has so many benefits. Try an easier variation like the cable lat pulldown or assisted pull ups to work your strength up to the point you will be able to do a complete bodyweight pull up.
A common variation of the pull up is the chin-up, where you hold your hands with a underhand grip (palms facing your body). Both variations work the same muscles, but the chin-up allows greater involvement of the biceps and and the chest to assist the motion, while the pull ups isolates the back muscles more. So for most people chin-ups will be easier to perform.
Keep your form strict rather than utilizing momentum to power yourself over the bar. Doing pull ups with a leg whip, and a jerking motion might feel easier and allow you to do more reps, but it will not train the proper muscles and will decrease your strength gains. It will also increase the risk of injury.
Keep the shoulders down and back. Shrugging the shoulders at the upper part of the movement will place unnecessary stress on your rotator cuff, traps and other surrounding muscles while minimizing the stress on the lats, this is not what you want. Focus on pulling your shoulders down, and keeping them there for the entire movement.
Focus on pulling your elbows, not your hands. As with any pulling exercise that you perform for your back, you should always focus on pulling the resistance using your elbows. This simple trick will help you keep the pressure where you want it, on your lats, while minimizing the stress on your arms. Imagine your hands and forearms are merely hooks that attach you to the bar. Instead of thinking about pulling your body upwards towards the bar, focus on driving your elbows down toward the floor instead.
Don't place the hands too wide. The hands should be positioned shoulder width apart or just a little more. Having a wider grip will not result in wider lats, but might cause shoulder damage.
Use the full range of motion. It is important to remember that the pull ups are a back exercise. Doing partial reps doesn't allow your lats to work properly which encourages the arms to do most of the work. Finish the reps with arms almost straight but not locked at the bottom, and your chin over the bar at the top.
Don't let your elbows flare too forward. Try to keep the elbows under the bar, and focus on driving them towards your back. This will keep the pressure on your lats instead of the biceps, and will keep you in a better mechanical position making it easier to pull the chest up.
Keep your body slightly tilted backwards. In order to perform a good pull up the shoulders need to be retracted and set to engage the posterior muscles. To hit your back properly during pull ups, think of raising the rib cage toward the bar, and arch the back a little.