In the figure to the left XL is inductive reactance, XC is capacitive reactance, R is resistance, and Z is impedance. The two kinds of reactance, inductive and capacitive, nullify each other. Inductors resist changes to current flow and capacitors resist changes to voltage. Reactance is different than resistance, but like resistance it is measured in ohms. Resistance resists the flow of electrons through a material, not changes in the voltage or current.
Reactance depends upon the frequency and the inductance or capacitance.
Impedance of a circuit is important. It is the resistance to both the flow of electrons and to changes in voltage or current. In a resistor impedance is purely resistive because the reactance equals zero. Impedance for a capacitor or inductor is purely reactive because the resistance equals zero. Resonance is the condition in which the combined impedance to two alternating circuits cancel each other out or reinforce each other to produce a minimum or maximum impedance.