The rack and pinion mounts to the cross member under the engine, and connects the steering wheel to the front tires. It is the center point connecting the steering column to the tie rod assembly, which then connects to the spindles or steering arms.
This assembly converts the rotational motion from the steering wheel into the linear motion (Left-Right) of the wheels. The Rack is a flat bar with notched teeth connected to the tie rod assemblies, and the Pinion is a round gear connected to the steering shaft. As you turn the steering wheel, the rack is pushed in the opposite direction.
Rack and pinions are used on newer applications and can be manual or power assisted. The power-assisted rack uses a power steering pump. This pump is mounted in the engine compartment and is belt driven.