You lose your body's shock absorber. Cartilage, the slippery material that covers the ends of bones, gradually breaks down.
One example is what can happen to your knees when you're overweight. The extra pounds put more pressure on the cartilage as it gets squeezed between the bones. It gets damaged and wears away, so there isn't as much left to cushion the joint.
The damaged cartilage makes movement painful. You may hear a grating sound when the roughened cartilage on the surface of the bones rubs together. You may get painful spurs or bumps on the end of the bones, especially on fingers and feet. The joint lining can get inflamed, but it's not common with osteoarthritis.