Also known as "CV joints", constant velocity joints are the flexible part of the axle that connects the transmission to each wheel to transfer power equally, while allowing the up and down motion of the suspension and the turning motion of the steering. As the name suggests, the CV joint provides consistent and regular velocity, regardless of the operating angle of the joint, ensuring there is no cyclic fluctuation between the speeds of the input and output shafts. Used on many rear, front, and four wheel drive vehicles, they can be found on all cars with independent suspension.
Covering the CV joints, you will find rubber CV boots. The boots act as an insulator for the joints, and are used to keep dirt out and grease in. The grease inside the boot is used to lubricate the joint, and is placed there by the factory.
CV joints should be serviced about every 50,000 miles. They require a special type of high temperature, high pressure grease, and you should never use ordinary chassis grease or multipurpose grease. The most common symptom of a failing CV joint is a clicking or clunking sound from the side opposite of the direction you're turning.