What is frequency response and why is it important in control systems?

Frequency response describes how a system responds to sinusoidal inputs at different frequencies - specifically the gain (amplitude ratio) and phase shift as functions of frequency. It is obtained by substituting s = j*omega in the transfer function. Frequency response is important because: it characterizes system bandwidth, it predicts stability margins, it guides controller design, and it can be measured experimentally. Bode plots, Nyquist plots, and Nichols charts are common frequency response representations.