XForms is a graphical user interface toolkit for X based on the X11 Xlib library. It allows you to create windows, containing all kinds of widgets (buttons, sliders, browsers, menus etc.) with a few lines of code and then attach actions to the widgets, i.e., have some function called when a button is pressed. To make this even easier XForms comes with a program called fdesign that allows you to design a GUI for a program directly on the screen and which then writes out the necessary C code for it.