The Markdown language was created in 2004 by John Gruber with substantial contributions from Aaron Swartz, with the goal of allowing people “to write using an easy-to-read, easy-to-write plain text format, and optionally convert it to structurally valid XHTML (or HTML)”.
Taking cues from existing conventions for marking up plain text in email such as setext, the language was designed to be readable as-is, without looking like it's been marked up with tags or formatting instructions, unlike text which has been formatted with a Markup language, such as HTML, which has obvious tags and formatting instructions. Markdown is a formatting syntax for text that can be read by humans and can be easily converted to HTML.
Gruber wrote a Perl script, Markdown.pl, which converts marked-up text input to valid, well-formed XHTML or HTML and replaces left-pointing angle brackets ('<') and ampersands with their corresponding character entity references. It can be used as a standalone script, as a plugin for Blosxom or Movable Type, or as a text filter for BBEdit.
Markdown has since been re-implemented by others as a Perl module available on CPAN (Text::Markdown), and in a variety of other programming languages. It is distributed under a BSD-style license and is included with, or available as a plugin for, several content-management systems.
Markdown is used in GitHub, GitBook, Reddit, Diaspora, Stack Overflow, OpenStreetMap, and many others.
Even this book is written using Markdown: Raw content of this page.
A markdown document is a text file with the .md extension. You can open a markdown file using a simple text editor.