Markdown is nothing but normal text with some formatting informations for headers, bullet lists, links, highlighting and so on.
A good overview on Markdown is provided by this Markdown cheatsheet.
HTML is the language for structuring a webpage. A webpage basically is boxes in boxes in boxes, which means the structure is coming in form of a tree. HTML is defining this tree.
HTML is the language for structuring the tree. CSS is the language for styling the elements of this very tree, its branches, its leaves and all of its elements. In CSS you can define the element's sizes, distances, fonts, colors, and so on.