FV-QR
Lets create a visual since math is fun!
Rotary (DeWalt, Makita, etc.)
Spins on one axis. This makes it harder to control on the paint surface because it 'pulls' in the direction the pad spins.
The angular velocity creates higher friction along the outside edge of the pad making cutting into the paint faster (the outside of the pad needs to travel further in 1 RPM than the inside of the pad). Smaller pads make a rotary easier to control but also puts more pressure on a smaller contact patch. The entire contact patch on a single axis is always in motion on the paint, or doing work. Learning to use a rotary properly is a skill that is learned over time.
Random Orbital (PC)
Spins on two axes minimizing pull and, in turn, friction on the paint surface creating a epicycloid pattern.
Only the central axis is driven while the secondary axis follows the path of least resistance. This is why they are safer for less skilled individual because it is practically impossible to burn through paint on a random orbital polisher. If you put 30 pounds of pressure on the machine it will almost 'vibrate' in place as the secondary axis follows the primary axis to keep the pad stationary.
All Random Orbit machines have a very large diameter for the secondary axis compared to the primary axis. The above image just an example.
Another term incorrectly used in detailing machines is 'Dual Action' (myself included). In a Dual action machine, both of the axes are driven. There is not a detailing machine (that I'm aware of) that drives both primary and secondary axes.
Modified by Mechorg at 5:26 PM 8-14-2008
