Simple plain-text markup language whose processor produces HTML.
Markdown is a markup language originally created by John Gruber to simplify writing HTML by using a simplified syntax. A markdown processor turns this into full fledged HTML (or in the case of MultiMarkdown, also into OpenDocument, LateX and more). Widely used among the more technically minded, both writers and services, because of its plain-text foundation, good readability and simple input. See the Wikipedia article for general reference.