I think he means different types of Windows OS.
I'm not a programmer, but I think that when you write code you need to consider the Graphic User Interface (GUI) you want to use to 'skin' your program. Problem is that if you go for the latest GUI options you often leave older OS'ses behind.
Say you program ONLY for Windows XP with the GUI DLL's provided by Microsoft, then you got a good chance that it will not work on Windows 98 and probably look awful on Windows 2000. So you need to consider the common default and add extra code to add extra skinning features depending on what OS the user runs.
And then there's the fact that whatever skin he codes, there's always someone who wants to use a custom one.
