Analog switch integrated chips (ICs), then turned on, will conduct both analog and digital signals from the input pin to the output pin. Digital switches can only accept digital signals and duplicate the logic level on the input pin at the output pin. When the digital switch is turned off, it returns to a default logic state.
The long version
An analog switch acts like a solid-state relay (i.e., with no moving parts). Analog switches can also isolate devices at their terminals when the analog switch is OFF. When ON, they conduct both analog and digital signals, regardless of the direction in which the signal is traveling. The switch control input for both kinds of switches is usually a digital signal input (called an input select), although other control triggers can be used to make specific applications easier to implement.
Analog switches are able to pass or isolate both analog and digital signals, but digital switches can only pass or isolate digital signal lines. Both are used instead of mechanical switches for convenience, reliability, and their small size as compared to mechanical switches.
There are some limitations on the signals that can be carried by both types of switches. Analog switches have a frequency response limitation due to channel capacitance. (A signal-level change can be caused by parasitic capacitance at high frequencies, for example.) For digital switches, there is a maximum frequency that can be fed into the input of the digital switch, after which the switch’s output state will no longer reliably follow the input. Calculate the transmission rate of your digital signal by noting the rate of the digital signal’s rise and fall times. You must also account for any delay from when the digital switch’s control signal (which activates the switch) changes, and when it subsequently enables the output. (Expect to see a change in rise and fall times based on varying conditions.) With both types of switches, the switch’s datasheet will reveal the limitations.
Both types of switches are typically manufactured as integrated circuits, often in packages with multiple (individual) switches. Multiplexors are also switches, but act more like a train station with multiple trains feeding via railroad switches onto one track, and is a separate topic.
The remainder of this article concerns analog switches. For analog switches, the switching portion of the device is made up of a couple of transistors. In Figure 1, the transistors are MOSFETs; one is a P-channel MOSFET and the other is N-channel. MOSFETs make perfect switching devices and are often used in power applications.