WYSIWYG is a short form of what you see is what you get. In the digital sense, it applies to applications that enable you to see exactly what appears on a printed document to appear on the display screen. The opposite of these applications is word processors that cannot display graphics and fonts that are different on the display screen. Desktop publishing is very fond of WYSIWYG applications.
WYSIWYG's history began with referring to line breaks being shown accurately on display screens by any word processor. Eventually, these applications had to display different sizes of fonts, even when only one typeface was present on the display screen. Today to be counted as a WYSIWYG, many different graphics and typefaces must be able to be displayed.
There are varying levels of what you see is what you get applications. In regards to desktop publishing systems, outline fonts are used to print text and many of these use fonts that are bit-mapped when displaying documents. In doing so, what you see on the monitor, is not exactly what you get when you print the document. Another example is laser printers resolution as their dots per inch is at least 300, while the premium monitors have a dots per inch of only 100. The difference in dots per inch equates to colors appearing different upon printing and sharpness of graphics and text being of higher quality.