What is the difference between a volumen and an expression pedal?
An expression pedal or a volume pedal is essentially a potentiometer that is operated with the foot, allowing us to control some parameter and thus free our hands.
A volume pedal is made up of an audio input and an audio output and, through a potentiometer, reduces or increases the level of the signal coming out of the guitar. Its function, then, is to control an audio signal.
An expression pedal controls a certain parameter (which can be, for example, the repetitions of a delay) by connecting it to the input of an audio device, such as an effects unit. It is thus a controller.
Within the volume pedals we can distinguish between active and passive. The only difference here is that the former require power, either by battery or by connecting them to a power source, while the latter do not, since they are, in essence, a simple potentiometer that lets through more or less amount of signal: a volume.