Markdown / Markup
All built-in story formats use a form of markdown or markup.
John Gruber and Aaron Swartz created Markdown in 2004 with the goal to create a way of adding some extra symbols to text to easily convert it into HTML. Since then, it has become very popular with sites like GitHub supporting it for text input.
Snowman and Chapbook use a modified version of Markdown for styling text.
Harlowe and SugarCube use what is known as markup. To change the presentation of text, extra symbols are added to create visual effects like emphasis and a stronger emphasis. Most wiki software and sites like Wikipedia use markup to style text.
What's the difference?
Markdown is markup. It is a way of changing the presentation of text through adding extra symbols that have special meaning when other programs read it and convert it into a different format like HTML. The only real difference is that Markdown has a name whereas "markup" is a category of all languages that perform the same actions.