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The lower ball joint is the pivot point between the lower control arm and the spindle, or knuckle support. Ball joints on a car are the pivot points for the front wheels. This holds true for nearly all modern vehicles.

Ball joints allow free movement in 2 planes at the same time. This takes away the need for upper and lower pivot pins. Its predecessor, the king pins, could not do this. Ball joints also free up the center of the hub, so that it is not obstructed. This allows drive axles for front wheel drive, 4-wheel drive and all-wheel drive.

The cost of replacing a lower ball joint can be anywhere from 30 dollars all the way up to 300 dollars or more. Cost will depend on the application, availability and how many ball joints are worn out.

Suspension System Parts - Lower Ball Joints

Lower ball joints are used in some 4WD drive vehicles that have also solid axles. In the case of a solid axle application, the lower ball joint connects the knuckle to the axle. The lower ball joint consists of a ball stud that is mounted into a steel housing. The ball stud can swing and rotate inside the housing, which also allows the spindle or knuckle to pivot. Greasing the ball joint on a regular basis will greatly increase the life of the part.

Wikipedia also offers this additional information on how ball joints work.