Markdown vs. HTML
Markdown is another very popular markup language. It is commonly used for writing documentation for software projects, readme files on platforms like GitHub or as a format for posting blog articles. Markdown code is designed to be easily readable by humans. Also, many note taking applications support markdown (e.g. Notion.so, Obsidian or Logseq), so users write their notes in Markdown. A few of the most basic formatting options like bold or italic text are even supported in chat messaging applications like Whatsapp or Discord.
Similar to HTML, the idea is to write normal prose text and sprinkle in a few special characters so that the text is displayed in a separate view, formatted as bold, italic, or as a headline, etc. You can write it in a normal text editor, markdown editors or code editors like Visual Studio Code. The latter two usually have some kind of split-view where the markdown code can be seen in one panel next to the the formatted text in another one.
Here is an example on how Markdown code (left side) will be displayed (right side):
On the left side is a markdown file. The right side displays the file with formatting applied. The code editor "Visual Studio Code" supports editing and previewing of Markdown.
For comparison, the next image shows how the same formatting can be accomplished with HTML code:
In the image HTML tags are wrapped around text to define the meaning and formatting of the wrapped text.
<h1> Creates a heading, <h2> a smaller sub-heading and <p> a paragraph. <strong> is for bolded text, <em> to emphasize (italics) and <del> to strike-through.
Markdown Cheat Sheet
Below is cheat sheet linked which shows Markdown code examples and what the corresponding formatted text looks like:
Further mention worthy markup languages are AsciiDoc and LaTeX. AsciiDoc has basically the same use cases as markdown. LaTeX is for writing academic papers with proper typesetting to generate pdf files. Many people use the online editor Overleaf to write documents with LaTeX collaboratively without the need to install anything.
Additional "lightweight" markdown languages are listed in this Wikipedia article.
Pandoc is a tool for converting between different markup formats.