GUI-construction involves everything that is used to programmatically construct a custom graphical user interface under Mathematica.
Constructing a custom graphical user interface is easy with Mathematica. It involves control objects like
Manipulate objects, gauges (like
VerticalGauge, etc.), static GUI elements like
Frame and the structuring and layout of these building blocks (using the
Grid family). Questions about how to build small interactive softwares should use this tag.
Questions about general dynamic interaction and content (like
Manipulate) should use the tag dynamic or manipulate. Questions dealing specifically with dialog notebooks and windows may use the tag dialog-window.