Not to contradict Robbie Rhodes, but to merely re-state his description
in more down-to-earth terminology:

VIBRATO is a fluctuation in the PITCH of a note or tone; i.e. the pitch
wavers slightly above and below the base pitch.  This is the effect
employed by singers, violinists, cellists, etc. to "warm" their sound,
and is the effect found on most electronic organs.

TREMOLO is a fluctuation in the VOLUME of a note or tone; in other
words the tone gets louder and softer but the pitch is unchanged.  Some
singers and instrumentalists such as saxophonists use this technique,
although I personally don't care for the resulting sound.  The term
TREMOLO in an entirely different context refers to a reiteration
between two or more different pitches, such as when a pianist "rolls"
an octave in the bass, quickly alternating between the two notes for a
specified duration.

TREMULANT is the term commonly used for the wavering effect in pipe
organs, created by different means by different builders, but typically
disturbing the wind source in a controlled manner.  (At least one
manufacturer of theatre organs called this "VIBRATO")  These mechanisms
typically created an effect that combines both frequency and amplitude
modulation, in other words both pitch and volume, as did the Leslie
speaker (by means of the Doppler effect).

The "warbling" effect in some European street and fairground flute
pipes is, I believe, created by some mechanism within each pipe, and to
my ears is again a combination of vibrato and tremolo.

David C. Kelzenberg
Iowa City, IA