In this video two steering racks are taken apart to demonstrate how they work inside.
The steering rack on a car is responsible for turning the rotational motion of the steering wheel into a horizontal translation motion that push the wheels in and out via the tie rods. This results in the front wheels turning.
Rack and pinion steering consists of a housing, within which there is a tube, called the rack, with teeth. The teeth engage the input shaft that's connected to the steering wheel, and is called the pinion gear.
In this video 2016 Toyota Camry and 2016 Chevrolet Cruze steering racks are cut open to show their internal workings. The Toyota rack is a basic rack and pinon setup and is purely a mechanical mechanism, since its electric power assist comes from a motor mounted on the steering shaft above.
The Chevy steering rack has a servo motor and a computer that aids with the assist, and it comes as an assembly mounted to the rack itself. The steering angle input sensor sends a signal to the computer, sitting behind the motor. The motor is then turned on to allow an electric assist to aid in steering the wheels. It is connected to the rack via a recirculating ball type mechanism. The steering input shaft meanwhile, connects to the rack on the driver's side via a traditional spiral gear.