RWHP is used when people refer to power output at the wheels, in this case the Rear Wheels usually measured on some type of chassis dyno. This is useful for comparing modifications/tuning or comparing the output of different cars. Also the number tells you how much power you are getting to the ground, taking into consideration the power it takes to spin your drivetrain.
BHP (brake horsepower) is what is commonly refered to as flywheel HP. Factory engines are rated this way, and people generally use this to compare their cars to a factory car rating. Measured on a engine dyno outside of the car, it uses some type of brake to work against the engine. When the engine overcomes the force of the brake, the amount of power needed to do is measured and the engine rated as such.