GDK is the low-level library used by GTK to interact with the windowing system for graphics and input devices. Although you will rarely use GDK directly in application code, it contains all the necessary functionality to create low-level windows in the screen and to interact with the user with various input devices. GDK acts as an abstraction over various windowing systems, so that GTK can be portable to all of them: the X Window System (X11), Microsoft Windows, Mac OS X Quartz.

GDK enables you to access events from keyboards, mice, and other input devices. Implementations of widgets in GTK use this functionality, and translate the events into higher-level signals that can in turn be used from application code. For example, a GtkButton widget will track GDK_BUTTON_PRESS and GDK_BUTTON_RELEASE events, which come from the mouse, and translate them as appropriate into a GtkButton::clicked signal when the user presses and releases the button in the right location.

GDK also provides low-level routines to access drag and drop and clipboard data from the system. When implementing custom controls, you may need to access these features to implement proper user interaction behavior.

GDK provides other functionality which is needed to implement a complete graphical toolkit like GTK. Since GDK acts as a platform abstraction, allowing GTK to run under multiple environments, it provides an API for all of the system functionality needed by GTK. This includes information about multi-head displays, resolution and color depth, colormaps, and cursors.

You should use GDK whenever you need low-level access to the underlying windowing system, including low-level access to events, windows, and the clipboard. Using GDK for these tasks ensures that your code is portable and integrates with the rest of your GTK code. The simple drawing routines in GDK should generally not be used; these are a left-over from when GDK simply wrapped the windowing system's drawing primitives. Instead, you should use the extensive functionality provided by Cairo to draw high-quality 2D graphics.