Reactive loads store energy and then return it to the mains. Inductive loads store energy in the magnetic fields of the inductors. Capacitive loads store energy in the charged dielectric of the capacitors.
Going back to the definitions of inductance and capacitance. Inductance resists the change in current. Capacitance resists the change in voltage.
Lets ASSUME we have a power source that is creating a sine wave voltage. When you have a fixed resistive load, the higher the voltage goes, the higher the current. When the voltage decreases, the current decreases. The voltage and current track each other.
When you add inductance to the resistive load, the inductor will fight the change in current. It does not want the current to increase, or decrease, or change direction. The way it does that, is to store energy by creating magnetic fields. When the inductor can no longer hold the current constant, the current will start changing. The inductor still wants to keep the current from changing. If the voltage decreases and the current starts to decrease, the inductor will try to maintain the current constant by releasing energy from its magnetic fields.
When you monitor the current and voltage waveforms, you will see the current waveform lagging the voltage waveform. The inductance is effectively delaying the current.
This causes problems in power systems because this stored current does not do any real work. It is not used to make light or turn motors. Because it is current, you have to upsize conductors to handle it but you don't get anything useful out of it. It creates extra losses in the form of heat.
It is analogous to carrying all of your tools with you, even though you only need a screwdriver. It wears you down but you are not doing any useful work by carrying them around.
This extra energy just circulates around, traveling between the power source and the load.
The ratio between the real power (power that does stuff) and reactive power (power that is just going for a ride) is called Power Factor (PF). When the PF = 1.0 the load is resistive. When the PF < 1.0 there is some reactive power in the mix. If you have a load that contains inductance, the PF is "lagging". If the load contains significant capacitance, the PF is "leading".
Remember: Reactive power is not doing any useful work. It is there only for the ride.
