HTML

Description

A.k.a Hypertext Markup Language
HTML is the markup language of websites. Markup means that you give meaning to a structure, you describe in a manner machines can understand what you mean. You define elements like

headlines
parapgraphs
lists
links
images
boxes
...

These elements are called tags. Tags can be styled with CSS to build complex layouts by nesting them inside and alongside each other.

Example

<!DOCTYPE html>
<html>
  <head>
    <title>Page Title</title>
  </head>
  <body>

    <h1>This is a Heading</h1>
    <p>This is a paragraph.</p>
    <div>
      This is a box
      <div>Inside another box</div>
    </div>

  </body>
</html>

Learn more

Basics: https://www.w3schools.com/html/default.asp
Basics (de): https://www.mediaevent.de/html/
All HTML Tags: https://www.w3schools.com/TAGS/default.ASP