Consider an AC circuit with a single voltage source and any number
of resistors, capacitors, and inductors. Recall that ``AC" is an
acronym for Alternating Current and the phrase ``AC circuit" implies
that all voltages and currents in the circuit are sinusoidal. If the
frequency of the source is fixed, then a complete analysis in either
the time domain or the frequency domain is possible. In the time domain,
a differential equation is extracted from the circuit and solved. In
general, the order of the differential equation is equal to the number
of energy storage elements (L's and C's) in the circuit. A much easier
method is to solve the circuit using phasor analysis in the frequency
domain. The reason analysis is easier in the frequency domain is because
differentiation in time transforms to multiplication by 
. As
a result, an algebraic equation arises rather than a differential
equation. Algebraic equations are generally easier to solve than
differential equations.
If the frequency of the voltage source is varied, the impedance of each energy storage element changes, and the response of the circuit varies as a function of the input frequency. The frequency response of a circuit is a quantitative description of its behavior in the frequency domain.