Care to elaborate a little? Where does the input come from? Under what circumstances do you need the script to execute? What's the purpose of needing this pop-up? What kind of environment (desktop, networked, etc)?
There are several programs that can be used in scripting to pop-up simple information boxes. xdialog, kdialog, and zenity, to name the most common. It's the exact nature of how it's to be used that's the real question.