A horsepower is a totally arbitrary measurement – dreamt up by James Watt when trying to compare the effectiveness of his steam engines against the horses they were replacing. One horsepower was deemed to be the equivalent of one horse lifting 33,000 pounds over one foot in one minute on the surface of the Earth.
ther measures are just an attempt to modernise this Victorian measurement. Brake-horsepower is, in car terms, effectively the same as horsepower and simply indicates that an engine’s energy loss due to friction has been taken into account – though, because horsepower usually includes this anyway, they are effectively the same.