I've reacently discovered that many refer to a double cardan joing (basically two traditional U-joints back-to-back) as a CV joint.
But what we normally think of as a 'CV-joint' as seen in most FWD vehicles and IFS setups (like our Rangers) is a joint like Bob described. Normally it uses a series of ball-bearings that are in very accurately machined groves. The groves let the joint articulate while still transmitting rotational torque.
is an interesting discussion of U-joints vs CV's. It is specifically a discussion of joints for farm and work equipment, namely PTO drives off tractors, but the same applies to 4x4's. They also refer double cardan joints as 'CV-joints' for their discussion.
The important difference is that a CV-joint can tollerate a greater angle between the two shafts than a U-joint can. A U-joint will have massive wear, or more likely breakage if operated at large angles. CV's on the other hand can deal w/ greater angles. This is why solid axle vehicles can still use U-joints at the wheels. The suspension articulation has zero effect on the drive-train angle at the wheel. The only thing that changes this angle is the steering. Limiting the steering to < 15 or 20 degrees is usually not a problem. However for IFS, both the steering AND the suspension articulation result in a change in drive-axle to wheel angle. This compounding results in some much larger angles.. hence the need for a CV.
Now we could use these double cardan style CV's.. but that would cost a lot of money. The ball-in groove CV is far cheaper and lighter. Hence why you find it on almost all light-duty pickups these days.. inlcuding our Rangers.
is an even better discussion of traditional FWD automotive CV's.