Your car's power steering system is likely hydraulic. It will have a power steering pump and a steering box (or steering distributor). The pump will be located on or near the engine and likely turned by a belt, transferring engine power to the pulley to turn a shaft that pumps fluid to pressure.
That fluid will then travel to the power steering box (distributor) and be used to assist in turning the wheels of the vehicle.
This method of giving power assist to the driver was created in order to allow redundancy. If the power steering box or pump cease to work, the driver still has full control and can turn the wheels without them (though it's harder to do).
The Power Steering Box's Components
The box will be located at the "bottom" of the steering shaft. This will be in the engine compartment on the driver's side of the vehicle. The box will look like a heavy cylinder that bubbles out to surround the steering shaft (see photo above).
The hydraulic oil or fluid (power steering fluid) will enter the steering box under pressure from the pump. It will turn a screw-like rotor that is coupled to the steering shaft by gears. This assists the driver when turning the wheel.
The box is usually one piece and if replaced, will be purchased as one unit. Replacement involves removing the bolts that hold it to the frame (usually the cross-rack) and sliding it off of the shaft or removing a shaft end-bolt. Once the fluid lines are disconnected, the box can be replaced.
Most home mechanics are likely capable of this job themselves, but keeping the wheel alignment perfect is not an easy task and may require a professional.