WYSIWYG is an acronym for “What You See Is What You Get”. In computing, a WYSIWYG editor is a system in which content (text and graphics) displayed onscreen during editing appears in a form closely corresponding to its appearance when printed or displayed as a finished product, which might be a printed document, web page, or slide presentation.

Or so says Wikipedia…

WYSIWYG editors do a good job of optimizing the screen display for a particular type of output. For example, a word processor is optimized for output to a typical printer. The software often emulates the resolution of the printer in order to get as close as possible to WYSIWYG. However, that is not the main attraction of WYSIWYG, which is the ability of the user to be able to visualize what he or she is producing.

In many situations, the subtle differences between what the user sees and what the user gets are unimportant. In fact, applications may offer multiple WYSIWYG modes with different levels of “realism”, including…

A composition mode, in which the user sees something somewhat similar to the end result, but with additional information useful while composing, such as section breaks and non-printing characters, and uses a layout that is more conducive to composing than to layout.

A code mode, in which the user can toggle to a version that contains the extra HTML tags (like BOLD) and make more complicated changes. Of course, users are supposed to be able to simply use the editor in composition mode, but a code view is nice to have.