What is the difference between markup and markdown language?

Question

All readme file have extension .md. I think markdown is also kind of txt file but how to write markdown. ✌✌✌👏👏🔻
On the other hand HTML is renowned as markup language but is there any other markup language available. ✌✌✌👏👏👏🔺
🅿 Thanks in Advance.

Answer 1

https://daringfireball.net/projects/markdown/

Answer 2

I had no idea there was such a thing as mark down.