Jazz Organists live in a different dimension than 'keyboard' generalists. The Hammond, for example, provides you with nine drawbars that allow you to sculpt your sound in real time, in ways that are foreign to folks who lack these control surfaces. Your instrument doesn't just have two keyboards and a pedalboard; it also has a whole array of tone controls that deserve your attention!
One of the quickest ways to discern between a 'keyboard' player and an organist is the level of attention paid to drawbar registrations during a performance. When I first started playing the jazz organ, I'd set my drawbars where I liked them and leave them there for most of the tune; now I've learned to make frequent (if small) adjustments to account for different sections of the piece, fluctuations in mood, or even a guitar player who suddenly switches between a single-line solo and a thicker, blocked chord texture (see also: Wes Montgomery.)
Pictured at right: Stevens Irwin's "Dictionary of Hammond Organ Stops: An Introduction to Playing the Hammond Organ." This book will tell you more about drawbars than you ever thought possible. (Available at various online retailers)
Most of us don't have these registration numbers memorized, but we go by the overall shape of the drawbars as they're configured. The tool below should give you an idea how various registrations correspond to the visual representation of the sound being sought.
When you play a single note on an organ, you can pull out different drawbars (stops?) to sound more than one note at a time. Unlike a piano, which plays a single tone for each key pressed, an organ key can sound several concurrent tones, often at different octaves and volumes.
The drawbars on the Hammond are closely related to the lengths of pipes on a traditional organ, and follow the overtone series. The octave of the tones produced by pulling out a single stop (with a volume level of 0-8, marked on the slider) corresponds to the "length" of a windblown pipe. The leftmost drawbar (16') is known as the "suboctave" and therefore plays an octave below the fundamental of whatever pitch you're actually playing. The 8' drawbar (the "unison") represents concert pitch--the same pitch level as what you are playing. The 2' (Blockfloete) drawbar is an octave above, and so on. The diagram below should illustrate this more clearly. Note that the "odd pipe lengths" (such as 5 1/3', 2 2/3', etc) will produce tones either a fifth or a third above (plus one or more octaves) the pitch you are playing. The farther out you pull the drawbar, of course, the louder the tone will be.