A Demo is a small self-contained script that has an audio or visual component, and whose purpose often is to showcase the capabilites of the Tcl language or of the script author. Demos are usually fun.
KPV and RS are currently the kings of the Tcl demo scene. KPV's demos, which are enumerated on his page, tend to focus on graphics, while the demos of RS tend to highlight marvels of the Tcl language and the Arts and Crafts of Tcl-Tk Programming, and of the fundamentals of programming in general.