Power factor basically is the portion that how effectually the power being supplied by the source is used. In simpler words, it can be said that it is the fraction of power being used (real power) to the total power (apparent power).
Power Factor = True Power(kW) / Apparent Power(kVA)
Power Factor = how much beer(kW) / per glass we buy(kVA)
Real power is actually the power that makes the equipment do valuable and industrious work while apparent power is a vectored addition product of the real and reactive power.
The value of power factor lies between -1 and 1. When the circuit is entire of inductive nature the value for PF is 0 and is said to be lagging and when the circuit is of the resistive type the PF becomes 1 and is called unity, and if the circuit is capacitive in nature the power factor becomes leading.
In mathematical terms, the power factor is basically the absolute value of the cosine of the phase angle of apparent power