The drive Axle is the component that connects the transmission to the wheel. they come in many different styles. The most common has an inner cv joint (constant velocity), a shaft and an outer cv joint. The cv joint allows for steering and suspension movement. The cv joint is packed with grease to reduce heat and wear. A boot is installed from the cv joint to the shaft, so the grease stays in and dirt stays out. The most common failure to a drive axle is the boot(usually caused by age and fixing), when the boot is torn the grease gets out, dirt gets in and damage to the moveable parts occurs.