X is the client and server for rendering pixels on to a screen, which is mapped to one of more monitors. It provides the most basic, bare bones way to draw a window to the screen, and that’s about it.

It’s not even a GNU/Linux thing, X predates Linux, it’s more of the standard system to draw a window on Unix-like systems.

X11 apps are GUI apps uncontrolled by/unaware of a desktop environment, theme engine, or window manager. They appear in a minimalist, maybe even brutalist, aesthetic. You can launch a single X11 app from the console, and the resulting window will be your entire graphical display/instance, returning you to the console after the program terminates execution.

Maybe another way to think about your question might be, think of X vs Wayland.

Interesting

> You can launch a single X11 app from the console, and the resulting window will be your entire graphical display/instance, returning you to the console after the program terminates execution.

Is this not possible with GUI apps using e.g. Qt or Electron?

You interpret X11 apps as a technical descriptor, but I think it's more a description of the ecosystem in which these apps evolved and whose culture they represent.

I see, thanks for explaining

Also most (all?) of these apps use xlib directly, rather than going through a toolkit like Gtk or Qt.

There are a lot of toolkits besides GTK and QT: XT, Motif, Xaw(3d), xforms, xview, wxwidgets.

A lot of these are built against Xlib directly, or X11-only toolkits such as Motif. Porting them to other window systems would require significant rewrites.