A jog dial, typically found on portable electronic devices
s, is a simple, one fingered interface.
Usually, it's a small scroll wheel, identical to one you would find on a scroll wheel-equipped mouse, except it usually is seated on the side of the device. This lets you hold the device with one hand, and use a single finger on that hand to operate the wheel. You can scroll up, scroll down, or press in. There appear to be two types; one that can scroll all the way around, like a mouse wheel, or one with a protrusion on the tip, meaning it can only go up or down one increment at a time. Both have their advantages.
Cellular phones and PDAs commonly feature this, as it enables you to browse listings, address books, etc. without using two hands or a stylus or having to use two or three buttons. You scroll up or down, and usually press inwards to invoke an action like select or dial.
It's handy for phones, you can look up a number single-handedly. PDAs use it to look up information single-handedly, some even use it to control video and audio players.
An example of a jog dial is here: http://www.clieplaza.com/en/product/sl10/jogdial.html
DejaMorgana says Hey. I could be wrong, but it seems to me you've got the wrong definition for jog dial. A jog dial, AFAIK, is one that doesn't rotate completely. If it does, it's just a plain old dial.