What device is a car radio dial?
In older radio's this was the variable capacitor that (in the common super type radio's) determined the frequency of the first stage and the oscillator, effectively selecting the single radio station you wanted to hear by analog means.

In modern radio's I guess it is a rotary encoder. You can tell by the subtle clicks you feel when you turn the dial. This rotary encoder is in effect two switches, that inform a microcontroller how the dial is turned. (This answer illustrates how a rotary encoder works.)