GTK is a library for creating graphical user interfaces. It works on many UNIX-like platforms, Windows, and macOS. GTK is released under the terms of the GNU Library General Public License, which allows for flexible licensing of client applications. GTK has a C-based, object-oriented architecture that allows for maximum flexibility and portability; there are bindings for many other languages, including C++, Objective-C, Guile/Scheme, Perl, Python, JavaScript, Rust, Go, TOM, Ada95, Free Pascal, and Eiffel.

The GTK toolkit contains “widgets”: GUI components such as buttons, text input, or windows.

GTK depends on the following libraries:

• GLib: a general-purpose utility library, not specific to graphical user interfaces. GLib provides many useful data types, macros, type conversions, string utilities, file utilities, a main loop abstraction, and so on. More information available on the GLib website.
• GObject: A library that provides a type system, a collection of fundamental types including an object type, and a signal system. More information available on the GObject website.
• GIO: A modern, easy-to-use VFS API including abstractions for files, drives, volumes, stream IO, as well as network programming and IPC though DBus. More information available on the GIO website.
• Cairo: Cairo is a 2D graphics library with support for multiple output devices. More information available on the Cairo website.
• OpenGL: OpenGL is the premier environment for developing portable, interactive 2D and 3D graphics applications. More information available on the Khronos website.
• Vulkan: Vulkan is the a newer graphics API, that can be considered the successor of OpenGL. More information available on the Khronos website.
• Pango: Pango is a library for internationalized text handling. It centers around the PangoLayout object, representing a paragraph of text. Pango provides the engine for GtkTextView, GtkLabel, GtkEntry, and all GTK widgets that display text. More information available on the Pango website.
• gdk-pixbuf: A small, portable library which allows you to create GdkPixbuf (“pixel buffer”) objects from image data or image files. You can use GdkPixbuf in combination with widgets like GtkImage to display images. More information available on the gdk-pixbuf website.
• graphene: A small library which provides vector and matrix datatypes and operations. Graphene provides optimized implementations using various SIMD instruction sets such as SSE and ARM NEON. More information available on the Graphene website

GTK is divided into three parts:

• GDK: GDK is the abstraction layer that allows GTK to support multiple windowing systems. GDK provides window system facilities on Wayland, X11, Microsoft Windows, and Apple macOS.
• GSK: GSK is an API for creating a scene graph from drawing operation, called “nodes”, and rendering it using different backends. GSK provides renderers for OpenGL, Vulkan and Cairo.
• GTK: The GUI toolkit, containing UI elements, layout managers, data storage types for efficient use in GUI applications, and much more.