Markdown is one of many ways, one of the better ones, of writing HTML without writing HTML. You use a more natural style to markup text and feed it to a Markdown conversion script which generates the corresponding HTML. Many sites on the net today natively (or via plugins) support Markdown markup (e.g: Tumblr). There are already a few cheatsheets (1, 2) for Markdown, but I made this one up anyway, since I wanted a different layout and something that fits in one A4 or Letter sized sheet of paper.
Below are standard (dark) and light versions, the latter more suitable for printing.
As with most of my projects, the sources (an Apple Keynote file; be warned: I am an utter amateur) are in GitHub and you are welcome to fork it for modifications.