A power cage (also known as a power rack, squat cage, or squat rack) is an item of weight training equipment designed to allow for a safe free weight workout using a barbell without the movement restrictions imposed by equipment such as the Smith machine. Originally described as an "apparatus for use with barbells," the power cage was patented in 1987 by Karl I. Mullen of Portland, Oregon.  As described in his patent, and paraphrased below:
It essentially comprises four vertical posts with movable horizontal bar catchers on each side. For strength the uprights are normally linked together top and bottom thus producing a cage. A safe free weight workout can be undertaken since dropping the weight will result in it being caught by the side bars.
A power cage can be used for a number of exercises including squat and various over-head presses. Partial bench presses, partial squats and partial deadlifts for instances are practiced in this cage. Partial movement allows for greater weight to be used in a safe way, allowing for an overloading of muscle groups to produce greater growth.
Although less experienced individuals are often seen doing so, curling in the power rack is generally deemed inappropriate.