Multiplexer: Multiplexers are circuits that can select one of many inputs.

4x1 Multiplexer: 4:1-Mux has 4 inputs with only 1 output. It has 2 data selector inputs namely S0 and S1, at which the control bits are applied. I0, I1, I2, and I3 represent the inputs bits. Only one of these will be transmitted to the output. But which one of the inputs will be transmitted will depend on the values of the controls.