WYSIWYG is an acronym that stands for "What You See Is What You Get". In the web design world, this refers to software that lets you develop a website visually. Programs like FrontPage, Adobe GoLive, Kompozer, and Dreamweaver are WYSIWYG HTML editors.
WYSIWYG editors can also refer to simple text boxes that are embedded in websites. These text boxes have controls that allow the user to change the styling of the content in that box.