Markdown vs markup - are they related?

• Markup is a generic term for a language that describes a document's formatting
• Markdown is a specific markup library: http://daringfireball.net/projects/markdown/
  These days the term is more commonly used to refer to markup languages that mimic the style of the library. See: https://en.wikipedia.org/wiki/Markdown

Mark-up is a term from print editing - the editor would go through the text and add annotations (i.e. this in italic, that in bold) for the printers to use when producing the final version. This was called marking up the text.

A computer mark-up language is just a standardised short-hand for these sorts of annotations.

HTML is basically the web's standard mark-up language, but it's rather verbose.

A list in HTML:

<ul>
    <li>Item one</li>
    <li>Item two</li>
</ul>

Markdown is a specific markup language, having its own simple syntax.

A list in Markdown:

* Item one
* Item two

Both of these will work in your stackoverflow posts.

Markdown can't do everything HTML can, but both are mark-up languages.

Markdown is a play on words because it is markup. "Markdown" is a proper noun.

Markup is just a way of providing functionality above plain text. For example: formatting, links, images, etc.