Github Flavored Markdown (GFM) is extension of Markdown by GitHub.
GitHub uses what we're calling "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.
If you're not already familiar with Markdown, you should spend 15 minutes and go over the excellent Markdown Syntax Guide at Daring Fireball. Note that markup that's not covered by GFM can't be supplanted with HTML.
Continue at https://help.github.com/articles/github-flavored-markdown .