Qpixmap hardware dependet, client - server store... What is this ?

I have read into the documentation :
Note also that QPixmap, unlike QImage, may be hardware dependent. On X11, Mac and Symbian, a QPixmap is stored on the server side while a QImage is stored on the client side (on Windows, these two classes have an equivalent internal representation, i.e. both QImage and QPixmap are stored on the client side and don't use any GDI resources).
What means client - server in this context ?
What I must to bear in mind ?
Thanks

Quote:

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Originally Posted by tonnot

What means client - server in this context ?

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Read about XWindow System.