What is horse power ? How is it related to the unit of power?

A horsepower is about 745 watts. James Watt, trying to
market his steam engines,  needed a way to explain to potential buyers how much work his
engines could do. He measured the work a horse could do in a given length of time, and
then did the calculations so he could explain his machines in terms of how many horses
one could replace. In 1993 RD Stevenson of the University of Masschusetts published a
Letter in Nature (364, 195, 17 July 1993) in which he reported that
under modern testing conditions, Watt's calculation was about right when compared to a
horse's sustained output.

If you wish to calculate the
horsepower involved in a specific motion, there are a number of factors that must be
measured, which vary depending on the situation - whether the motion straight line or
rotational is a big factor. For most cars, the horsepower is not actually measured, it
is calculated, either through measuring engine displacement (how much volume the pistons
move through), or indirectly by putting the car on a dynamometer, a device that measure
torque output at the tires.