Work = force x distance
Torque is the ability of the engine to do work about a rotational axis therefore torque is work.
Engines convert fuel into torque.
Power is torque multiplied by engine speed to produce a measurement of the engine's ability to do work over a given period of time.
Horsepower and torque are related. Torque is the measurement of how much work an engine does, while power is a measure of how fast the work is being done.
An engine is most efficient at its torque peak. During Torque peak (maximum efficiency) engines produce most of the desirable grunt work (torque) at lower engine speeds, which means reduced wear-and-tear and better fuel economy.