This is intended as a quick reference and showcase. For more complete info, see John Gruber's original spec and the Github-flavored Markdown info page.
This cheatsheet is specifically version of Github-flavored Markdown. This differs slightly in styling and syntax from what Github uses, so what you see below might vary a little, but it should be pretty close.
Also, check out the Markdown Cheatsheet that is not specific to MDH.