Markdown is a text-to-HTML conversion tool for web writers. Markdown
allows you to write using an easy-to-read, easy-to-write plain text
format, then convert it to structurally valid XHTML (or HTML).

Thus, "Markdown" is two things: a plain text markup syntax, and a
software tool, written in Perl, that converts the plain text markup to
HTML.

WWW: http://daringfireball.net/projects/markdown/