What is resonant circuit ?

A circuit that has a set capasitance and Inductance, will "Resonate" at a set frequency of AC Current and is called a resonant circuit. Resonace occurs when Capasitive Reactance ( XC= 1/2pieFC) and Inductive reactance (XL = 2pieFL) (L = Inductance in Henrys, C = Capasitance in Farads) are equal to each other. To understand how this really works, you need to know the effects of inductance and capasitance on an AC Circuit. Inductance changes the phase angle of an AC sine wave by increasing the angle, Capasitance reduces the angle. When AC meets a capasitance or inductance, it reacts with them, and produces a resistance that only an AC current at the given frequency would see. When the frequency reachs the resonate level (given by Fr = 1/2pie(LC)^1/2), the reactances cancel each other out and the AC current basicly ignores them and resistance is figured by the Ohmic resistance of the coil that provides the inductance. 

Circuits that use resonance in everyday use:
AM/FM recievers
Class-C Amlifiers
Metal detectors