GTK, or the GIMP Toolkit, is a popular open-source library used for creating graphical user interfaces (GUIs). It provides developers with tools to design and build applications that run on various operating systems, including Linux, Windows, and macOS. GTK is known for its flexibility and ease of use, making it a preferred choice for many software projects. Originally developed for the GIMP image editing software, GTK has evolved to support a wide range of applications, from simple utilities to complex desktop environments like GNOME. It uses the C programming language but also offers bindings for other languages, allowing developers to create cross-platform applications efficiently.