A CV half shaft, commonly referred to as a drive axle, is the component that transmits power from the differential directly to the vehicle's wheels. A regular two-wheel-drive vehicle will have two half shafts located in their respective engine compartments.
The CV half shaft, or constant velocity half shaft, is comprised of two joints, boots and clamps. The components must be properly greased in order to remain functional.
The most common reason for a CV joint to fail is a torn boot. The boot houses the bearings that allow the half shaft to spin, and when the boot tears, the bearings lose their grease and are exposed to environmental debris.