Development Tools - Editors, IDE's, etc.
"Standard" markdown. * Syntax
GitHub uses "GitHub Flavored Markdown," or GFM, across the site--in issues, comments, and pull requests. It differs from standard Markdown (SM) in a few significant ways, and adds some additional functionality.
If you're not already familiar with Markdown, take a look at Markdown Basics. If you'd like to know more about features that are available in issues, comments, and pull request descriptions, such as task lists, read Writing on GitHub.