HTML is a markup language that defines the structure of your
content. HTML consists of a series of elements, which you use to
enclose, or wrap, different parts of the content to make it appear
a certain way, or act a certain way. The enclosing tags can make a
word or image hyperlink to somewhere else, can italicize words,
can make the font bigger or smaller, and so on.
CSS is the language for describing the presentation of Web pages,
including colors, layout, and fonts. It allows one to adapt the
presentation to different types of devices, such as large screens,
small screens, or printers. CSS is independent of HTML and can be
used with any XML-based markup language.
JavaScript is a scripting language that enables you to create
dynamically updating content, control multimedia, animate images,
and pretty much everything else. (Okay, not everything, but it is
amazing what you can achieve with a few lines of JavaScript code.)