An active buzzer will generate a tone using an internal oscillator, so all that is needed is a DC voltage. A passive buzzer requires an AC signal to make a sound. It is like an electromagnetic speaker, where a changing input signal produces the sound, rather than producing a tone automatically.
To identify them, if you apply a DC voltage to them and it buzzes, it's an active.
As far as commands go if you want to control the pitch, you would need a passive buzzer. PWM on the Arduino can be used to control the pitch and the volume at the same time (which may or may not be what you want). If you wanted to change just volume or just pitch I suppose some external circuitry would be required to change the amplitude without changing the voltage, and vice versa.
A couple sources:
Source, Source, and Source, as well as @Peter Bennett's comment.