BHP and RWHP are two very different things, but a lot of people are confused as to their true meaning. Let me start with RWHP and then we will get around to BHP.
In the case of an automobile people are generally interested in knowing the power made by the engine and the power available at the drive wheels. Engine power is measured at the crankshaft/flywheel and this is often refered to as flywheel hp. Drive wheel power is measured at the rear wheels for a rear-wheel-drive car (RWHP), at the front wheels for a front-wheel-drive car (FWHP) and at all four wheels in the case of an all-wheel-drive car (AWHP). Because of the losses involved in transfering power from the engine to the drive wheels, flywheel hp is always higher than the drive wheel hp. This loss is categorized as driveline loss. It generally ranges from 15-20% depending on whether the car is a 2 wheel drive (FWD or RWD) or a 4 wheel drive (AWD).
HP measurement for cars is normally carried out by first measuring torque and then computing the associated power. Torque measurement can be performed using either a "brake dynamometer" or an "acceleration dynamometer." In the first case a force (torque) is applicated by means of a brake to keep the engine speed a constant. This force is exactly equal to the torque produced by the engine at that speed. Measurements are carried out a intervals of 250rpm to obtain a torque graph for the entire rpm scale. Using this torque curve power can be computed and this power is known as brake-horsepower or BHP. The second type of dyno, measures the rate at which a drum of fixed mass is accelerated by the engine. From this data the system computes the acceleration torque and then power. This way of measuring power should be known as acceleration HP. While either type of dyno can be used to measure flywheel HP or drive-wheel HP, it is generally found that brake dynos are employed to measure flywheel HP and acceleration dynos are used to measure drive wheel HP.
Both brake HP and acceleration HP have their own place. Brake HP gives a true indication of the ability of an engine to perform constant work (say keep the car at 175mph for the entire afternoon as is the case with the NASCAR cruisers). An acceleration dyno, on the other hand, is more useful if your intention is to determine a car's ability to accelerate - i.e. fly down a drag-strip. Note that unlike BHP, acceleration HP takes into account the effect of reciprocating mass and rotational intertia. However, I will not get into a discussion on that unless folks are interested.
Hope this discussion was of some help.