GTK and Qt: the two toolkits

In short: GTK and Qt are libraries with which programs draw their windows, buttons and menus. GNOME programs use GTK, KDE programs Qt. Both run on every desktop.

In more detail: A toolkit provides ready-made controls. The choice determines appearance and behaviour (dialogs, file picker, fonts).

In detail

• GTK (GIMP Toolkit): written in C, originally for GIMP. Currently GTK 4. LGPL licence. Programs: GIMP, Firefox (on Linux), Inkscape, GNOME applications.
• Qt: written in C++, Qt 6 current. LGPL/GPL licence or commercial. Programs: VLC, OBS, Telegram Desktop, Krita, KDE applications.
• Matching the look: with themes for both toolkits (Kvantum for Qt, GTK themes) programs can be shown uniformly.
• Libadwaita: GNOME library that gives GTK 4 programs a uniform modern look.
• Others: Electron (web technology, e.g. VS Code), Flutter, wxWidgets, FLTK, iced (Rust).

See also: GNOME, KDE Plasma