Here's the explanation:

CV-Joint stands for Constant Velocity Joint. The CV joint is part of your axle. The old fashioned rear drive cars have the drive axles in a housing. The housing is held to the chassis through shocks, panhard arms, and/or leaf spring mounts. This assembly is also known as a solid axle setup or live axle.

You can't use this in the front of a car because the front wheels also steer. In a front wheel drive (or AWD in this case) you have independent front suspension that improves handling and allows the car to steer comfortably. In order to use independednt suspension, you have to use 3 (in some cases four) piece axles. The three pieces are the inner bearing/shaft that goes into the differential; the main shaft; and, the outer bearing/shaft that goes into the wheel bearing.

The parts between the shaft and the bearing/shaft assemblies are your CV Joints. They are covered by a grease filled boot. The CV Joints look like smaller driveshaft Universal joints that are used to connect a driveshaft to axle in a rear wheel drive car. These joints allow the wheel to move with the suspension and be driven by the differential at the same time.

The CV Joints usually go bad from 2 things: being over-stressed or broken CV Boots. Overstress could be from too much power going through the axle or lowering or raising a vehicles suspension beyond the axles' range. Usually, the axles would have to be shortenned or lengthened to keep the correct geometry when altering the ride height. Too much power is obvious but it could also mean too much dumping of the clutch, also.

A torn boot allows the grease to escape so the joint has no lubrication. The effect is the same as draining the oil out of your engine: either something is going to seize or break. A torn boot also allows contaminants such as water or dirt to enter the joint. This pushes the grease away from the critical areas and has the same effect as loosing the grease.

The first symptom is usually the clanking or clicking around turns. Sometimes you may not hear the first symptom or it just doesn't appear. Then you have the second symptom, the joint just breaks and you're not going anywhere. This is not fun. I learned this lesson the hard way a few years ago. It was only about 3 miles from when I first heard the clicking before the axle broke. You can also check for a really bad axle by grabbing it when the car is parked and trying to move it. It should have no play in it at the joints. If it has some play to it, have a mechanic check it.

I hope this helps to answer any questions on what a CV Joint is.