In graphical computer software a panel is :

• A widget commonly packaged as part of a Widget toolkit
  Widget toolkit
  In computing, a widget toolkit, widget library, or GUI toolkit is a set of widgets for use in designing applications with graphical user interfaces...
  
   for a graphical user interface
  Graphical user interface
  In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
  
  . See toolbar
  Toolbar
  In a graphical user interface, on a computer monitor, a toolbar is a GUI widget on which on-screen buttons, icons, menus, or other input or output elements are placed. Toolbars are seen in office suites, graphics editors, and web browsers...
  
   and dialog box
  Dialog box
  In a graphical user interface of computers, a dialog box is a type of window used to enable reciprocal communication or "dialog" between a computer and its user. It may communicate information to the user, prompt the user for a response, or both...
  
  

• A folder
  Folder
  Folder may refer:* File folder, a kind of folder that holds loose papers* Folder , a virtual container within a digital file system, in which groups of files and other folders can be kept and organized...
  
   in some graphical operating system
  Operating system
  An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
  
   interface
  User interface
  The user interface, in the industrial design field of human–machine interaction, is the space where interaction between humans and machines occurs. The goal of interaction between a human and a machine at the user interface is effective operation and control of the machine, and feedback from the...
  
  s that provide tools for configuring the system and the drivers. See control panel
  Control panel
  Control panel may refer to:* Control panel , a flat, often vertical, area where control instrumentation is mounted.* Control panel , the tool in the operating system which allows most or all of the settings to be changed through a user interface** Control panel ** Control Panel ** Web hosting...
  
  .

Panels in widget toolkits

Panels in widget toolkit

Widget toolkit

In computing, a widget toolkit, widget library, or GUI toolkit is a set of widgets for use in designing applications with graphical user interfaces...

s often have no specific graphic characteristics, but are mainly used to group children widgets

Widget (computing)

In computer programming, a widget is an element of a graphical user interface that displays an information arrangement changeable by the user, such as a window or a text box. The defining characteristic of a widget is to provide a single interaction point for the direct manipulation of a given...

 together. They allow to have a better control on the layout of the widgets.

For example, in this XUL

XUL

In computer programming, XUL , the XML User Interface Language, is an XML user interface markup language developed by the Mozilla project. XUL operates in Mozilla cross-platform applications such as Firefox...

definition, two horizontal hbox panels are enclosed in a vertical vbox panel, alongside with two buttons. This allows to be sure that the label and the textboxes for the Login and the Password fields are correctly aligned.