Markdown is a lightweight markup language, allowing people “to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML)”. An excellent Markdown Syntax Guide is by Daring Fireball. Sites such as GitHub, reddit, Diaspora, Stack Overflow, OpenStreetMap, and SourceForge use Markdown to facilitate discussion between users. GitHub uses “GitHub Flavored Markdown” (GFM) for messages, issues, and comments. It differs from standard Markdown (SM) in a few significant ways and adds some additional functionality.