Here's a break down comparison on how a ball joint works, and the differences between the OEM and aftermarket design.
A ball joint is a suspension component that is used where three-axis rotation is required. Normally it would connect the control arms to the steering knuckle of the car. On a McPherson front suspension setup, there is one lower ball joint, while on a double wishbone front suspension setup, there's an upper and lower ball joint.
As a result, ball joints can wear down over time and have to be replaced. Failure to replace it can lead to the suspension dismembering itself, and the wheel to come apart from the vehicle.
This video opens up an OEM ball joint and an aftermarket and takes a look at the differences in components inside. The OEM ball joints typically use a metal ball stud with a plastic bearing, while the aftermarket ball joint use a metal stud with a sintered metal bearing.
However, the OEM ball joints are typically sealed, non serviceable components, while the aftermarket ball joints come with a grease fitting. A sintered metal bearing must be periodically greased for it to outlast an OEM ball joint.
Differences in the material, construction, dimensions, coating and design are also discussed in this comparison.