Just to add to @MattL's answer...
Great and Swell are usually assigned to different manuals. (Although they can be linked by couplers.) The Great manual will usually be assigned to principal stops, or as this page describes it, the Great manual usually:
contains the meat and potatoes of the organ: the principal chorus.
The Swell manual will be linked instead to pipes which are enclosed in a box, which the organist can open or close using a foot-pedal, in order to change the volume of the sounds played on this manual. As this pedal allows changes of volume, it can also be called the expression pedal. Also, the sounds played by the Swell manual are more likely to be accompanimental, softer or solo reeds.