The WYSIWYG or What You See Is What You Get is a text editor that allows a user to type and immediately watch the text resulting in styled like bold, italic, underline or strikethrough fonts, as you can imagine the main functionality of the WYSIWYG is to ease the creation of rich text; is commonly used in blogs text editor or the creation of documents like google docs.
At the time of the writing of this entry the most popular WYSIWYG libraries are: