Powerlifting is a strength sport that consists of three attempts at maximal weight on three lifts: squat, bench press, and deadlift. As in the sport of Olympic weightlifting, it involves the athlete attempting a maximal weight single-lift effort of a barbell loaded with weight plates. Powerlifting evolved from a sport known as "odd lifts", which followed the same three-attempt format but used a wider variety of events, akin to strongman competition. Eventually, odd lifts became standardized to the current three.
The deadlift being performed by 2009 IPF World Champion Dean Bowring
A squat is a strength exercise in which the trainee lowers their hips from a standing position and then stands back up. During the descent, the hip and knee joints flex while the ankle joint dorsiflexes; conversely the hip and knee joints extend and the ankle joint plantarflexes when standing up. Squats also help the hip muscles.
Bodyweight squat
Kirk Karwoski completing a heavy squat in competition. Note the spotters either side of him and the power cage which the barbell was lifted out of.
A barbell pad reduces pressure from the steel barbell on the back. This may reduce discomfort. Some pad designs are straight and some have an indented centre to further reduce the risk of direct pressure to the spine.
The hack squat as pictured page 70 of George Hackenschmidt's book The Way to Live (1908)