A torque tube is a sollid connection between the transmission and the rearend. There are no visible u-joints, and you would not see the torque tube spin while the vehicle is in motion.
An open drive rearend is what we are typically used to dealing with on modern rear wheel drive vehicles. A drive shaft with visible u joints can be seen spinning while the vehicle is in motion.
A torque tube rearend is much more efficient at transmitting the torque than an open drive rearend, so more of the available horsepower will make it to the wheels. This is in part because the tube prevents the axle from rolling and maximizes the effeciency of the joints.
The down side to the torque tube is parts availability. There are few gearsets that will fit in a torque tube rear, and most of them are not suitable for modern interstate driving. There are also few transmission choices to use with that torque tube rear. The torque tubes were designed for trucks rated for less than 150 hp... in the case of most 1/2 ton chevys with the torque tube, they were rated for less than 115 hp. When modern 300 + HP v8s are used in these trucks the torque tube transmission/and or rearend usually do not last very long. Most commonly one of the concealed u-joints breaks, as this tends to be the weakest link.
