From what I know.. there are two types of dynos, the inertial dyno, and the load dyno.
Dunno exactly how they work, but from what I know, the inertial dyno is (usually) the single roller type (there are duals, but most are single, I think!) and the load dyno is usually the dual roller type (for each front/rear wheel).
Each manufacturer of dynos have their own way of determining horsepower (complex maths... of calculating drum size, friction loss, etc. etc. I don't understand at all!!) so dyno's from different makes will in fact give different readings.. (dunno how much exactly, but shouldn't differ too much, since the there are only a few variables that might cause different readings.. drum size, weight, etc.) But I don't really know for sure.
For a load dyno, it needs to be calibrated for every car, since due to tire widths and possible pinching of the tires, there is a possible drag factor (which is why it should be calibrated every time a different car gets onto the rollers.) It works by applying load via giant brakes (you usually can see them at one end of the rollers) so you can keep a constant rpm (at different throttle positions) and play with fuel and ignition settings.
For a inertial dyno, apart from (usually) having one roller.. I have no idea how it works, so you're on your own there.
But I do know for sure that dyno results can be skewed or twisted to show higher than normal performace numbers... really depends on how 'honest' the operator is.
Although it shows different numbers, in the end, it's just a tool to help tuning with less emphasis on hp numbers. (imagine trying to tune at 120kmh on public roads, with a helper strapped in front of the car/in the passenger seat trying to read/change the numbers of the computer screen while the outside guy checks for any engine abnormalities (dipstick flying out, etc. etc) as the car speeds past a turn... ROFL!)
