MD files are likely Markdown files, which are simply plain text files. They typically contain formatting that can be easily interpreted by other programs to create formatted text (mostly used on the web) while still being human-readable.

MD files can be a variety of things, but they are most likely plain text files that use the Markdown convention. The files are plain text and look like this: *This would be bold text* while _this would be italic text_. There are more examples of course, but the text remains human-readable and contains enough information for programs to understand what formatting to apply.

A .MD or .MARKDOWN file can be a Markdown Documentation file. It is a plain text file that uses the Markdown language to describe how the document should be formatted. README.md is a generic MD file that contains textual instructions.

SEGA Mega Drive ROM files also use the MD file extension. They are a digital representation of a physical game from the SEGA Mega Drive console (called the SEGA Genesis in North America). Emulation software uses the MD file to play the game on a computer.