Markdown is a way to style text on the web. You control the display of the document; formatting words as bold or italic, adding images, and creating lists are just a few of the things we can do with Markdown. Mostly, Markdown is just regular text with a few non-alphabetic characters thrown in, like # or *.
You can use Markdown most places around GitHub:
Gists
Comments in Issues and Pull Requests
Files with the .md or .markdown extension