Here in no particular order are some key terms related to the organ. I am not going into much detail here, because a lot of terms related to the organ don't really come up when related to organs depicted on stamps. There are plenty of other sites that can give more information about the organ itself. Check the links at the bottom of the page.
Console: the place where the organist sits, the keyboards and other control devices. Most organs will have 2, 3, or 4 keyboards on the console, plus a pedalboard played by the feet. In general each knob represents a rank of pipes.
Facade: Very often the only part of the organ one sees. It's the front of the case which holds all of the pipework and other mechanical portions of the instrument.
Pipes make the sound. Wind is blown through the pipes to generate tone. There are reed organs in which air is blown over reeds to make the sound, and electronic organs which have no pipes. I am not addressing those in this blog at all.
Rank: a set of pipes making a particular sound. There are 61 notes on most modern organ keyboards which means one pipe for each, of every variety of sound the instrument makes. There are compound ranks (mixtures) in which each key has 2, 3, 4 or more pipes associated with it.
Stop: in general the same as a rank, though in the case of compound ranks, a single stop will control the sounding of several ranks which compose the particular sound. Sometimes one counts stops to describe the size of an organ, other times one states the number of ranks (which can be considerably higher if there are several mixture stops on the organ). Organists will state both when they want to be precise: "34 stops which include 41 ranks" for example which in this case would mean that nine ranks of pipes are part of mixture stops. More details would be needed to know how exactly the ranks are distributed.
Manuals: the keyboards. These are played by the hands. The pedalboard is played by the feet. Most organs will have 2, 3, or 4 manuals on the console. Very small instruments and positivs and portativs will have just 1. The console of the organ at Atlantic City Music Hall in the USA has 7 manuals!
I use three terms frighteningly indiscriminately. Renovation, rebuild and restoration each mean very particular things to the people engaged in such work. They all have to do with executing repairs on the instrument, or making additions to the number of ranks included, or reworking the mechanical systems in part or in their entirety. Renovation often refers to work that will modernize or otherwise enhance the mechanical systems. Restoration usually refers to turning back the clock as it were on additions or changes that have failed or have outlived their usefulness or are out of character with the overall make-up of the instrument. Rebuild can refer to either of the first two terms or to work done after an accident has harmed the instrument in some way.
Positiv: a small organ of 1, 2 or perhaps as many as 6 ranks. These are movable, if not quite portable.
Portativ: an even smaller instrument that can be set upon a table, or even carried about.
Got a terminology question not answered here? Leave it in a comment and I'll reply back there.