VGUI2

VGUI (or, technically, "VGUI2"), is used to display dynamic 2D images in Source. This includes dialogues and buttons, the HUD, and even the surfaces of monitors and screens in the 3D world. It is platform-independent and Unicode-compliant.

VGUI2 forms can technically be displayed on the Windows desktop (e.g. Steam) as well as in-game, but mod authors don't have access to that functionality.