Deadlift

The deadlift is one of the best and a core exercise for building muscle mass and density across the back and legs.The deadlift is one of the best back strengthing exercises it's also a good workout for your legs, it is included in most of the proper workout routines.Any athlete seriously involved into the fitness franchise performs this exercise, almost every certified personal fitness trainer will include this exercise into his clients workout programs. When carried out properly, the deadlift can bring serious progress and fast muscle gain. But improper technique or trying to lift too much weight can lead to serious injuries.
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It is advisable to first master the proper technique of the deadlift exercise and then increase the weight. It is important to keep good form even on warm-up sets. For proper implementation of the deadlift some flexibility is needed (especially the Achilles tendons,the muscles on the back of the thighs and buttocks),  if  there is an absence of such a flexibility it is recommended to stretch before performing the exercise.
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With heavier weights you could use weight lifting straps, as in most cases the weight will cause the forearms to fail before the back therefore the back won't be stressed enough. A weight lifting belt can be used as well, such a belt adds additional stability to the lower back. But its use should be limited to heavy weight sets to prevent the muscles from getting acustomed to the weight lifting belt.
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The width and the type of the grip are important factors for muscle loading, so choose an option that is the most convenient. You should have your feet roughly shoulder width apart.
The ordinary deadlift recruits the erectors, glutes, quadriceps, back of thighs, lateral and traps muscles and forearms.personal trainer certification schools

Muscles Involved

Target 

• Erector Spinae

Synergists

• Gluteus Maximus
• Adductor Magnus
• Quadriceps
• Soleus

Dynamic Stabilizers

• Hamstrings
• Gastrocnemius

Stabilizers

• Trapezius, Middle
• Trapezius, Upper
• Levator Scapulae
• Rhomboids

Antagonist Stabilizers

• Rectus Abdominis
• Obliques

Variations

• Deadlift with legs straight (Roman deadlift) - Recruits the lower back, but the main stress falls on the back of the thighs and buttocks. Here the weight is usually lighter than the ordinary deadlift. This option can be performed in leg workouts, after leg curls. 

Deadlifts with straight legs can be: classical and roman. In classical one powered the weight hangs freely throughout the whole exercise (when bent) it is 50 to 60 cm in front of your feet. Classical deadlifts causes more strain on the lower back because the center of gravity is shifted forward.
Therefore, the preferred option is a straight leg Roman deadlift. In this variation the weight is near your feet throughout the exercise and you're not allowing it to swing, even in the lower position. So you have to work a little with your hands steering. Roman deadlifts put less strain on the lower back. Whether it's Roman or classical - the muscles involved are the same. The difference is only in the lower back straining.

• Deadlift type "Sumo"(sumo deadlifts) - is widely used by power lifters. Legs wide apart, feet pointing outwards. Thus the main load falls on the legs and buttocks with less involvement of the back. Grip is narrower than normal

• Upper (partial) deadlift - only the upper part of the range of motion is performed, this variation is oriented toward a greater load on the upper back muscles and traps.

• Deadlift with dumbbells - technique is like a regular deadlift. This variation is easy to implement.

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Method of execution

Preparation

With feet flat beneath bar, squat down and grasp bar with shoulder width or slightly wider over hand or mixed grip.

Execution

Lift bar by extending hips and knees to full extension. Pull shoulders back at top of lift if rounded. 

Slowly and controlled return to the starting position.Repeat.

Recommendations

• Deadlift can significantly burden the waist, especially in with heavier weights and improper form. Until you get used to the movement, it is advisable perform it with light weights.
• Perform the exercise slowly and precisely, without swinging the body. Keep your back straight throughout. Otherwise, you will easily get your injury or the lower back and the spine that are strained significantly and are very vulnerable. If you can not keep the weight with your back straight, lower the weight.Leave your ego out of the gym when performing this exercise.
• Look forward throughout. If you bend or lift your head high there's a risk of a spine injury, though not as great as a  rocking body and a curved back.
• Do not use your arms to pull the weight up. Your hands should just be holding the bar nothing more arms should be inactive.
• If you're performing a variation where you're bending your legs,bend them to the point where your thighs and lower legs are at a straight angle(90 °). Do not bend legs further as you turn it into a squat exercise.
• Do not lock out your knees. This will put unnecessary strain on your knees, and you will lose muscle load. Stretch legs almost, but not entirely.
• If you're performing the straight leg variations, don't lean too much, especially if you use heavy weights. Some bodybuilders even prefer to perform the deadlift on a bench so they can go lower. But the more you bend, the more you strain your lower back.
• Always warm-up well, especially before heavy sets. The load on the muscles and joints will be very large, and in can easily lead to an injury.

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Relevancy

The deadlift is an excellent exercise, developing strength and mass on the lower back and hamstrings.You can see them in tons of bodybuilder training videos, included in most of the personal trainer programs it's an amazing exercise. It may be beneficial to bodybuilders at any experience level.
However since it is a dangerous exercise,it's best not to implement it in your training if you are a beginner. You can "try" deadlift workouts occasionally, with light weights.
Include them into your workouts only after you've gained some knowledge and experience in the gym.
Do not do them if you are recovering from injury or you're prone to such.
Where in your leg workout you will choose to perform it it's your choice.
It's usually in the end of back training workouts(because lower back strain) or leg workouts (because of the strained tensors, and hips).
If performed right the deadlift can transform your body.
(sources:exrx.net)
(pictures:thedeadlift.com)