By hvhipot 
Impedance is the resistance to the flow of current in an AC circuit, measured in ohms. The reactance in a DC circuit without induction or capacitance. However, there are resistance, inductance, and capacitance reactance in AC circuits. All of these that flow in the opposite direction of the current and are opposite to the direction of the current are called impedances.
It is important to remember that any circuit can be simplified as a resistor, which can be a single inductor or a capacitor. Reactive circuits cannot have both inductors and capacitors at the same time. The effect of inductive reactance on current is the main reason for current attenuation, where current lags behind voltage. The effect of capacitance reactance is to guide the voltage through the current. These two effects are opposite. Therefore, circuits are either capacitive or inductive, but not both.
Remember this:
L represents inductance – L represents hysteresis. Complex circuits have many resistors, capacitors, and inductors that can be simplified into single resistors and single resistor reactances, whether capacitive or inductive. The following circuit is a series RLC circuit. Because the inductive reactance is 100 ohms and the capacitive reactance is 50 ohms, the net reactance of the circuit is 50 ohms.
When XL=XC
When a circuit contains exactly the same number of inductors and capacitance, the net reactance is zero, and the circuit is resistive. Therefore, if the series resistance of a circuit is 100 ohms (or with an inductive reactance of 200 ohms and a capacitive reactance of 200 ohms), the impedance of the circuit is either 100 ohms or 100 ohms.
Resonance in a circuit: Any passive circuit, if it has an inductor, will resonate, while the resonance characteristics of a capacitor are determined by the input voltage and current in phase. Impedance (or admittance) is completely true when this condition exists. Basically, there are two types of resonant circuits:
(a) Series resonance.
(b) Parallel resonance.