Any circuit contains active components is non-linear, as an example power switches like semiconductors are no-linear, let say dc-dc converter is a nonlinear because it has IGBT power switches to transfer the energy between the circuits, transistors are another type of active components so when they are placed in the circuit, they make it exhibit non-linear behaviour, diodes sometimes are considered non-linear, The circuits that contains passive components like resistors, inductors and capacitors are considered linear So to make it easy if you see the V-I characteristics, so you can figure out if the component is linear or non linear Hope this has helped