Continuous-time systems have continuous input and output signals, with time measured continuously, and are generally defined by differential or algebraic equations.

For instance, in an RC circuit, the relationship between input and output voltage is expressed through a differential equation derived using Ohm's law and the capacitor's voltage-current relationship.

Discrete-time systems process input and output signals at specific intervals and their behavior is described by difference equations at distinct time instances. An example of such a system is a simple model for the balance in a bank account from month to month.

Another type of system is time-varying and time-invariant.

A time-varying system has parameters that change over time. Continuous-time, time-varying systems are described by differential equations with time-dependent coefficients, while discrete-time, time-varying systems are described by difference equations with time-dependent parameters.

Time-invariant systems produce an output that when the input signal is time-shifted, shifts identically without altering the system's characteristics.

An RC circuit is time-invariant if the resistance and capacitance values remain constant. If these fluctuate, the system becomes time-variant as the experiment results will vary depending on when the experiment is conducted.