Feedback control systems are categorized as linear or nonlinear, time-varying or time-invariant, and classified by signal types as continuous or discrete-data systems.

Linear systems are theoretical models used for simpler analysis. An amplifier in a control system behaves linearly within certain signal ranges.

Physical systems inherently exhibit nonlinearity. The on-off controller in missile systems is an example of how nonlinearities can be intentionally incorporated to enhance performance.

Time-invariant systems have constant parameters, unlike a motor's winding resistance, that varies when the motor is first being excited.

Time-varying systems have changing parameters like a missile's mass decreases as fuel burns during flight in a guided-missile control system.

Continuous-data feedback control systems, like in the heated jacket, use signals as functions of continuous time. DC control systems use unmodulated signals, while AC control systems use modulated signals to reduce noise and disturbance.

Discrete-data control systems use pulse trains or digital code signals. This makes them resistant to noise and more efficient in terms of space and flexibility.