In graphical computer software a panel is "a particular arrangement of information grouped together for presentation to users in a window or pop-up." In ISPF, a panel is "a predefined display image that you see on a display screen.".:p.16
Panels in widget toolkits often have no specific graphic characteristics, but are mainly used to group child widgets together. They provide better control on the layout of the widgets.
For example, in this XUL definition, two horizontal hbox panels are enclosed in a vertical vbox panel, alongside with two buttons. This insures that the label and the textboxes for the Login and the Password fields are correctly aligned.