Electric pianos emulate their acoustic ancestors in a variety of ways. Sometimes an electric piano is very similar to an acoustic piano, all the way down to the pedals. Other times, an electric piano has significantly different components to allow for greater sonic diversity.
Biased Switches
Biased switches are switches that have a spring mechanism that maintains the switch position if it is not being pressed down. One type of biased switch is the "momentary" switch, which is only engaged when the user presses it down; a sustain pedal is a type of momentary switch.
Foot Switches
"Foot switch" is a general term for any switch that is typically activated by foot. This can include both momentary switches and non-momentary, or "latch"-type, switches.
Appearance
A proper sustain pedal for a digital piano is often modeled after the sustain pedal of an acoustic piano. A foot switch that is used for sustain functions is mechanically similar, though it can be very different in appearance.
Polarity
Most sustain pedals and foot switches have a control to toggle the polarity of the switch itself. Normal polarity typically allows you to trigger the function by pressing down on the switch; reversed polarity allows you to disengage the function by pressing down on the switch.
Flexibility
Both sustain pedals and other sustaining foot switches can be used to control any function that might be controlled by a momentary switch. Consider the appearance and potential application of the switch before making a choice; sustain pedals are typically used to simulate the experience of sustaining an acoustic piano, whereas other foot switches accomplish the same goal but do not simulate the acoustic piano experience.