There are a number of differences between an organ and piano, besides the obvious form and function issue. Both use keyboards as their primary means of performance, but the mechanics behind those keyboards are completely different. A piano is generally considered a member of the percussion musical family, while an organ falls in with the woodwinds, electronic and even the brass families. The main difference between the function of the two during performance is percussion versus air power.
A piano generates sound whenever the performer strikes a key attached mechanically to a hammer. In turn, this hammer strikes at least one metal string held in tension on a large frame. The various strings are tuned to specific frequencies, which allows the performer to create chords and dissonant sounds by striking more than one key at a time. These vibrations may continue for a short amount of time, but eventually the performer must restrike the keys to maintain a piano's sound.
An organ's keyboard, on the other hand, does not physically strike anything. Instead, the performer on a modern electronic organ completes a circuit whenever he or she depresses a key. The keys are also tuned to specific frequencies, but the individual notes can be held for an indefinite period of time without the need to restrike the keys. This is one major difference between the percussive piano and the electronic organ — a piano can only sustain a note for a short period of time, while an organ can hold it indefinitely.
Another difference between a piano and an organ is their functions as accompanying instruments. Since a piano is essentially a percussion instrument, much of its initial power comes from the first strike or attack on the keyboard. This means the accompanist for a church congregation or choir can quickly establish both a rhythm and a melody line. It is not unusual to hear a player start off an introduction to a song, followed by the other instruments. A piano, much like a guitar, is a good leading instrument.
An electronic organ, by comparison, is more of a follower than a leader when accompanying singers. It achieves much of its performance power during the sustain, not the initial attack on the keys. One of an organ's primary functions in a band is to fill in the sounds not covered by the more percussive instruments. Accompanists may not even attempt to play the melody line, preferring instead to play supporting chords while the pianist provides guidance for the vocalists.
Another difference between the two instruments is the number of sounds each can generate. A piano's keys can be altered somewhat to create a tinny honky-tonk sound, or the overall size of the instrument can create richer tones, but it is designed to sound like a piano. An organ, by comparison, can be altered through the use of pipes or electronics to sound like woodwind, brass and reed instruments. An organ also contains several different levels of keyboards, called ranks, which can all be set for various combinations of instrumental effects.
Some keyboardists develop a preference for either the piano or the organ over time. Each calls for a slightly different set of manual dexterity skills and musical theory. Organists must often learn to play bass notes on a pedaled keyboard, while simultaneously controlling the dynamics with a swiveling volume pedal. Pianists must learn complex chords and fingerings while also learning the physicality of a percussion instrument. While both a piano and an organ are keyboard-driven, the differences between them can be quite a challenge for musicians.