Complicated GdkWindow

[Nengming Zhang <znm china com>]

I'm a new Gtker, I'd like to know the deferences between the following
four _GdkWindows, where and when to use them, why not the other.   It
seems it easy to program in Gtk when I read some examples.
typedef struct _GdkWindow             GdkWindow;
typedef struct _GdkWindow             GdkPixmap;
typedef struct _GdkWindow             GdkBitmap;
typedef struct _GdkWindow             GdkDrawable;

A GdkBitmap as a mask is somewhat confused me.
GdkBitmap *mask = NULL;