For many years, the much-ballyhooed invention of the wheel has overshadowed the most likely, near-simultaneous invention of an equally important modern innovation: the axle. Without the axle, the wheel would not do anyone much good. Rocks can roll as well and, while this may be great for music, it's not very useful for transportation purposes.
While the wheel has remained more or less round since its inception, the axle has evolved to spin in all manner of configurations and situations. Insofar as automobiles are concerned, there are more than a few different types of axles. Some are of the short and stubby variety like the spindles upon which non-driven wheels spin. Things get more complicated, however, when the axles have to deliver power to the wheels. If a powered wheel had nothing to do but roll on a perfectly smooth surface, the solution would be simple. A straight axle with no suspension would do the job. As anybody who has recently driven around can attest, roads are everything but perfectly smooth; thus, the axle must be able to deliver power to the wheels and travel with the suspension as it soaks up the bumps.
This gets particularly tricky in the case of a front-wheel-drive car where the wheels are steering the vehicle as well. With all the angles created by the suspension traveling over the bumps and wheels steering to and fro, the axle must be as flexible as a gymnast while spinning like a whirling dervish. The solution was to make axles with constant velocity, or CV joints. Front-wheel, all-wheel and certain rear-drive vehicles with independent suspensions also use this same type of axle, which employs two CV joints to deliver flexible power to the wheels. Some vehicles even employ CV joint axles in the drivelines.
Get the Boot.
A CV joint is a collection of bearings and cages that allows for axle rotation and power delivery on a number of different angles and planes and for the axle to change length as it travels up and down. All the parts spin around in a protective layer of grease that is held inside the joint by a flexible, rubber boot. Without the boot, the grease would simply spin out of the CV joints. The boot also keeps dirt and grime out. On the CV axle are two CV joints along with one or two sets of splines. One side of the axle gets power from the engine, the other delivers the power to the driven wheel. Presto.
Over time the rubber boots of the CV joints can become cracked, torn, or otherwise compromised. Once moisture and dirt get into a CV joint and grease makes its way out, the CV joints days are numbered. Signs of impending failures are a click-click-clicking or metallic crackling noise while turning and accelerating or a clunking upon deceleration. While one can repack, reboot, and rebuild CV axles, it makes more sense for the average do-it-yourselfer to procure replacement units and swap out the old axles. If a boot is torn or missing, it is possible to save the joint if caught early enough, but usually the damage has already been done. Replacing or servicing the entire CV joint axle assembly may be the smarter move since replacing a boot may involve removing an axle, which may also involve removing half or all of the vehicle's brakes and suspension.
While the basic concept of a CV joint axle is the same from vehicle to vehicle, there are a number of different peculiarities that make access to a service manual an important first step before disassembly. Axle pullers and impact wrenches may be required. Pry bars, circlips, and snap rings inside differentials may also be part of the bargain. This knowledge is best gained before getting started, not while 90-weight gear oil is dripping down your sleeve.