The back squat is the most basic strength exercise in weightlifting, and one of the most commonly used exercises other than the competition lifts.
Place the barbell behind your neck—retract your shoulder blades tightly and rest the bar in the meat of your upper traps. Place your feet between hip and shoulder width with the toes turned out so that at full depth each thigh and the corresponding foot are in line with each other. Set your back in a complete arch, take in a large breath, and lock it in, forcefully tightening all trunk musculature. Bend at the knees and hips simultaneously to move down as directly as possible into the bottom of the squat with an upright posture, maintaining tension on the legs throughout the movement and controlling the speed of the descent. Full depth is achieved when the knees are closed as much as possible without losing the arch in the back (if you cannot sit into a full depth squat, you need to work on mobility). Upon reaching the bottom position, immediately transition and stand as aggressively as possible, again with the knees and hips together to maintain your upright posture—try to lead the movement with your head and shoulders.
For a lot more information on the execution of the back squat, and squat in general, see the following articles:
Plenty of disagreement exists on how the back squat should be performed, but little within the competitive weightlifting community.
The back squat is the most effective exercise in weightlifting for building basic strength, particularly of the legs and trunk. It is used for general leg strength development primarily, although nearly always is used in combination with the front squat for weightlifting.
There are a huge number of possibilities when it comes to programming the back squat. Most commonly weightlifters will use sets of 1-5 reps, but it’s not unheard of to use as many as 10 occasionally, briefly and far out from competition.
The back squat can be performed as a pause squat, with prescribed tempos (usually slow eccentric movement), and as a 1¼ squat.