An wind instrument that produces sound by forcing air through flue and reed pipes mounted on a wind chest. Traditionally used only in churches, the organ is slowly losing appeal in favor of praise bands and the piano.
Most people have not been exposed to good organ music, which is an incredible shame, because the organ is an extremely expressive, diverse, sensitive, powerful, and unique instrument.
An organ consists of usually two or more manuals (ie. keyboards) which include all the notes from low C two ledger lines below the bass clef to somewhere around A, two and a half octaves above middle C. Larger organs will have as many as five of these. Each has a name, the main manual is the Great, other names include Swell, Choir, Positive (that's with a long 'o'), and Solo.
The Swell manual is particularly interesting: its pipes are always enclosed in a wooden box, called the Swell Box, with shutters on one side which can be controlled by a pedal. This serves as a method of gradual dynamic changes.
Each manual has its own set of stops; a stop is simply a set of pipes, one for each note on the keyboard. An organ will have anywhere from 8 to 150 or more of these. They are combined to make more complex sounds, so that when you push a single key, as many as 20 or 30 pipes can play at the same time. Stops are labeled with a name indicating the type and timbre of the stop, and a number indicating the pitch at which the stop speaks. An 8 foot stop speaks at the unison pitch; it's called an 8 foot stop because the lowest pipe (low C) is about 8 feet tall. Likewise, a 16' stop speaks an octave lower, a 4' stop speaks an octave higher, and a 2 2/3' stop speaks an octave and a fifth higher.
One of the especially distinctive sounds an organ can make that hardly finds a parallel anywhere is that achieved by adding mutations. Mutations are stops that speak at a pitch other than octaves from the pitch being played. The most common are 2 2/3' (octave and a fifth), 1 3/5' (two octaves and a major third) and 1 1/3' (two octaves and a fifth). When combined with an 8' stop, the net effect is that every time you play a note you also hear a note that is, say, an octave and a fifth higher. Now if you try this on the piano, it sounds mighty strange, but on an organ it is pleasant and unlike the sound of any other instrument.
Organs get their great diversity from the ability to combine the stops in different ways. There are soft, soothing stops and there are loud, powerful, brass stops. An organ can be comforting one moment and mighty the next.
There are many differences between playing the piano and playing the organ. Besides the obvious addition of the pedals, there are two characteristics of the organ that make it unique. First, there is no sustain pedal, so in order to hold a note, you must keep your finger down on it. This leads to different kinds of fingering involving substitution and single lines that will alternate between hands. Also, there's no way to accent a note on the organ, so you have to produce the illusion of accent in order to maintain musicality. Methods of doing this are explained in The Agogic Principle.
Most organs have pedals, which go from low C to around F above middle C. It has it's own set of stops too, but they're usually lower (more 16' and sometimes even 32' or 64'), since the pedal is usually used to reinforce the bass of chords.
An organist wears special shoes to help facilitate playing the pedals. They look like dress shoes, but they have an uncommonly high heel (about 1 inch) and they have leather soles, to make sliding around easier. They are basically tap shoes without the tap.
The organ is a very rewarding instrument to play. Make all the cracks about little old ladies you like, but I can't think of any instrument capable of producing the volume an organ does (hint: electric guitar doesn't count).