What is an axle?

​Answer: an axle transfers power to the wheels and allows for a difference in speed between wheels.  That's right when you make a turn the right wheel and left wheel on the axle have to travel at different speeds because they are traveling on two different length arcs and have to arrive at the end/straight ahead position at the same time.  Take a look at these tire tracks from one car making a slight turn

What's a u-joint?

Answer: a u-joint allows the drive shaft to transfer power through changing angles as the axle moves up and down with the suspension.

Question-What is a drive shaft?

Answer-A drive shaft is the link between the transmission and axle. The axle is linked to the wheels and follows the contour of the road just like the wheels.  The transmission rides on the vehicles suspension or springs and is isolated from the contours or bumps of the road.  This means the drive shaft must transfer power to the axle through changing angles and lengths. 

Drive shafts and Axles