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).

"Markdown" is two things: a plain text markup syntax, and a software
tool, written in Perl, that converts the plain text markup to HTML.  PHP
Markdown is a port to PHP of the original Markdown program by John
Gruber.