Here's the problem. Manufacturers claim a slightly higher HP number on thier cars from the factory. So people had to distinguish a cars useable HP by calling it WHP.
The way they get away with this is they give you the HP number they get on an engine dyno. This number would be called Crank HP. Vehicles loose some power in the driveline; the transaxle (FWD) or transmission and driveshaft (RWD) or transfer case (AWD).... oh yeah, and transaxle from a Mid-engine Rear wheel drive car such as the NSX or MR2 (as well as most supercars in which the engine is directly behind the driver but in front of the rear wheels)

There is a certain percentage of power loss that can be gathered for each layout, but I think it's going to usually be about 15 to 30 HP for most cars. Give or take a little here and there.

So. A Honda Civic Ex that is rated at 120 Max HP from the factory is only going to be able to put down about 100 HP to the road. And remember that number is it's maximum HP number. You will not achieve Max HP in normal driving. It would take you revving the car up to it's redline which I think is about 6800 to actually put out that much power. So that particular car will only (usually) be good for about 85-95 useable HP.

Some people may correct me on a few small things but that should give you a good idea of that stuff.