The Hammond tonewheel organ, first introduced by Laurens Hammond in April 1935, is the one against which all contenders are measured. For this reason its technology is outlined below, and not the technology used in more recently manufactured (non-tonewheel) Hammond organs.
Hammond organs were originally intended to be an affordable substitute for Pipe Organs. While today, no one would consider them a serious contender in that market, their distinctive sound is popular (indeed, sought after) in nearly every style of Popular Music, particularly Jazz, Blues, and Rock and roll.
All Hammond console organs includes two 61-key manuals; the lower, or great, and upper, or swell, and a pedal clavier consisting of 25 keys. The concert models have a 32-key pedal clavier.
The secret of the Hammond tonewheel organ lies in its method of tone production. The tone generator assembly consists of an AC synchronous motor (Hammond made electric clocks before he made organs) connected to a geartrain which drives a series of tonewheels, each of which rotates adjacent to a magnet and coil assembly (similar to an electric guitar pickup). The number of bumps on each wheel in combination with the rotational speed determines the pitch produced by a particular tonewheel assembly. The pitches approximate even-tempered tuning, (it's done with integer (rational?) math after all).
A note on the organ consists of the fundamental and a number of harmonics, or multiples of that frequency. In the Hammond organ the fundamental and up to eight harmonics are available and are controlled by means of drawbars and preset keys or buttons. The setting at any particular time is applicable to one manual, either great or swell. (Harmonic content adjustment is provided for each manual independently.) The Hammond organ creates its tone colors through additive synthesis.
Other Hammond innovations which are important parts of the character of the Hammond Organ are vibrato, percussion and harmonic foldback. Key-click, originally an unwanted side-effect, has also become a sought after part of the "Hammond Sound".
Also see: HammondWiki:WhatIsAHammondOrgan.