Consider an ideal alternating current (ac) circuit consisting of a source and a generalised load,
where both the current and voltage are sinusoidal. If the load is purely resistive, the two quantities reverse their polarity at the same time, the direction of energy flow does not reverse, and only real power flows.
If the load is purely reactive, then the voltage and current are 90 degrees out of phase and there is no net power flow. This energy flowing backwards and forwards is known as reactive power.
Since there is no any power loss when current flows through the pure inductor or capacitor, but the
energy is stored in the form of magnetic field(in inductor) or static field (in capacitor). So the reactive
power is the power used for magnetising or is stored in capacitor in the form of static field.
Actually no any system is loss less. So active power is the power that is lost in the form of heat in
the system or we can say active is the power used in the system to compensate for loss.