Axles are an integral component of most practical wheeled vehicles. In a live-axle suspension system, the axles serve to transmit driving torque to the wheel, as well as to maintain the position of the wheels relative to each other and to the vehicle body. The axles in this system must also bear the weight of the vehicle plus any cargo.

A non-driving axle, such as the front beam axle in heavy duty trucks and some two-wheel drive light trucks and vans, will have no shaft, and serves only as a suspension and steering component. Conversely, many front-wheel drive cars have a solid rear beam axle. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle.

In the former case, bearings or bushings are provided at the mounting points where the axle is supported. In the latter case, a bearing or bushing sits inside a central hole in the wheel to allow the wheel or gear to rotate around the axle. The axles serve only to transmit driving torque to the wheels; the position and angle of the wheel hubs is an independent function of the suspension system.

This is typical of the independent suspensions found on most newer cars and SUVs, and on the front of many light trucks. These systems still have differentials, but will not have attached axle housing tubes. They may be attached to the vehicle frame or body, or integral in a transaxle.

More Info: en.m.wikipedia.org