What is difference between a switch and a diode?

Both have two terminals, but a switch has a mechanical actuated element that closes the circuit. A diode does not have this. A diode blocks current flow when a reverse voltage (that is not too large) is applied to it, but when a forward direction voltage is applied current is allowed to flow; the cost is a small voltage drop across the diode. A switch doesn't care which direction current is flowing, and there should not be any voltage drop (ideally) in a switch.