CUA is Common User Access, a standard developed jointly by Microsoft and IBM back in the day when they were working on OS/2 together, as part of IBM's interface-paradigm-thing SAA (System Application Architecture). CUA defines several common interface behaviors for GUIs, allowing for a common, recognizable set of keyboard and mouse actions across the several platforms it was target to, including Windows, OS/2, and TopView. Most PC-based GUIs adhere to CUA standards to some degree.
Because the mouse was a not entirely common accessory at the time of the development of CUA, one of the goals was to allow graphical interfaces to be mouse-optional. So, general rules were created for keyboard behavior in a GUI that would allow those without a mouse to still make full use of their GUI applications. I consider this a major advantage of PC-based GUIs over, say, the Macintosh, which up until very recently had almost no keyboard equivelency for mouse actions.
Off the top of my head, I can think of the following behaviors defined by CUA:
- Alt-F4 to close top-level window
- Ctrl-F4 to close MDI child window
- Alt-Tab to switch between top-level windows
- Ctrl-Tab to switch between MDI child windows
- Ctrl-Esc to open window list (perverted in Win95 to open Start menu])
- Alt-Esc to change top-level window foucs
- Alt-F3 to minimize top-level window (not supported by Windows)
- F10 to move focus to menu bar
- Alt-(underlined letter) to open menu
- Alt-Space to open system menu
- Double-click title bar to toggle between maximixed and normal window
- Double-click system menu to close window
- Tab and Shift-Tab to change focus in controls
- In later years, CUA was extended to include support for more modern GUIs, such as Shift-F10 to open a context menu
- The behavior of the minimize and maximize buttons was defined, although their appearance was left to implementations
- CUA also defined GUI standards not specifically related to input mechanism, such as the fact that menu bars appear at the top of a window, windows have Z order (that is, there is a top-most window, and other windows have various "depths" beneath it), windows have icons associated with them
This is a far from complete list; according to strict CUA doctrine, almost any commonly-available user command had a keystroke shortcut. CUA standards don't get a lot of respect these days, Windows 95 played pretty fast and loose with the CUA standards, altough it still maintains the bulk of its keyboard support. Some Windows applications do in fact require a mouse; this is usually the fault of that particular application rather than the OS. It was up to the OS and applications to support the standard, although I think that very few really did so the fullest capacity.
OS/2's implementation of CUA is much more complete than Windows', however, and using only a keyboard, I can move much faster in that GUI than in any other.