Markdown is a lightweight markup language, originally created by John Gruber and Aaron Swartz allowing people "to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML)".The language takes many cues from existing conventions for marking up plain text in email.
Markdown files can be stored as
.txt, but you'll also find:
Having these clues as a file extension will let you easily transform these files into others. Markdown makes it very easy to convert these plaintext files into HTML, OpenDocument fiels, PDF, LaTeX or RTF using MultiMarkdown.
And, if you haven't noticed yet, StackExchange uses Markdown for formatting, too!