What is the difference between rdoc and md?

Answer fragment from another deleted Stack Overflow question by kiddorails:

RDoc is a fantastic tool which enables the automatic generation of formatted HTML containing our documented code.

For example, most of gems on RubyGems fetch documentation and compiles them to give the resultant rdoc.

If your project doesn't have README.md, GitHub may parse README.rdoc to display details. If it has both, it will use README.md, silently ignoring rdoc. I usually have both, README.md for Github, and README.rdoc for RDoc documentation on RubyGems.

RDoc - is a RubyGem which produces HTML and command-line documentation for Ruby projects. RDoc includes the rdoc and ri tools for generating and displaying documentation from the command-line.

Digging Deeper Rdoc

Markdown also known as md is a plain text formatting syntax5 designed so that it can optionally be converted to HTML using a tool by the same name. Markdown is popularly used as format for readme files, or for writing messages in online discussion forums, or in text editors for the quick creation of rich text documents.

Github's Markdown

Github's Markdown basics

RDoc is a markup language. Markdown is a different markup language. HTML is yet another markup language. Any of those can be used to markup text, and all of those are supported by various code-browsing tools including but not limited to GitHub.