The part known as a pitman arm converts the angular, turning motion of the sector shaft into the linear motion needed to steer the wheels.
A rack and pinion system eliminates the need for a pitman arm, since the steering wheel shaft leads right into the rack and pinion assembly, where a gear on the shaft rolls across the toothed rack directly.
The drag link (also called a center link) is what connects the pitman arm to the tie rods, which are connected via a ball joint to the steering knuckle. The movement of the pitman arm causes it to move left or right to turn the wheels in the appropriate direction, based on steering wheel input.
The pitman arm connects to the drag link through a ball joint. It this ball joint becomes worn, the gap will often cause loose, less responsive steering. The grease in the joint should be refilled fairly often (doing so with every oil change is a good increment for most vehicles) to prevent this wear from happening.