The type of back squat done in powerlifting is much different than a back squat you would do for training for sports or for fitness. In powerlifting, the goal is to squat as much as possible, so different techniques are used to increase the amount of weight lifted.
The bar is placed lower on the back, more towards the base of the rear delts, rather than the traps. Usually a wider stance is used in powerlifting, as well as pointing the toes outward, at close to a 45 degree angle. The hands should be in a position on the bar so you can have the bar in a the low position on the back. The closer the better, for the most part however. You should be looking out or up, not down.
When squatting, it is better to "sit back" rather than just squat down
to utilize the stronger muscles of the hips. If you sit back, your
shins should remain relatively purpendicular, at least so the knees do
not go over the toes. As far as depth, the rules of powerlifting
require you to get the top of your leg where it meets the hip below the
top of the knee.
For a more in depth look on the squat, check out Rickey Dale Crain's article on Advanced Powerlifting Techniques.