I've come across a piece of sheet music for an organ and I was just curious what the Grt. marking above measure 1 and 17 means and what the Sw. marking above measure 9 means. I've seen it before in a few other pieces for organ, but I'm not sure what it means.
They refer to divisions (manuals) of the organ: Grt. for Great (French 'Grand Choeur', German 'Hauptwerk') and Sw. for Swell (French 'Grand Orgue', German 'Schwellwerk'). For English, American and German organs, in a two-manual configuration, the lower manual is the Great, and the top manual is the Swell. French organs usually have the Swell at the bottom. You can find more information in this article.
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.
The 'grt', sometimes designated 'I', is the manual closest to the player on a two manual organ. It's called the great manual. The other is called the swell, or 'II'. Hence 'sw'. Different sounds can be made using the stops for each manual, and sometimes they are coupled, so playing one manual operates both, giving more sound options.