HTML is a markup language, which means it's for annotating the content of a document. Since the introduction of HTML5, these annotations have had a very specific purpose: to describe the structure and meaning of this content.
This is in stark contrast to older versions of HTML, and many other markup languages, which are concerned with the appearance of the content. A good example of this is Markdown, which is commonly used by instant messaging services and forums to change the appearance of text.
Our HTML documents are going to contain the main content of our pages, augmented by annotations in the form of HTML tags. This includes things like images and videos, forms, and of course the text content of our pages.
We'll be covering HTML in a lot of detail in the next section.
CSS is used to describe the presentation of a document. In other words, it's for changing how things look on our web pages.
Modern CSS is extremely versatile and powerful. We can even create complex animations with nothing but CSS.
CSS uses a system of selectors, which describe what we should change the appearance of, combined with property definitions, which describe how the appearance of that things should change.
We'll be covering CSS in section 5.