Like every seemingly complex scientific phenomenon, the best way to understand it is to break it down into little parts.So let us break down a website into two main parts; the Front and the Back end. Think of the Front end as what the user of a website sees or can see and the Back end as what the user can not see or have access to. Within the tech community, the Front end is also commonly known as Client side while the Back end is commonly known as the Server side.
In the most simplistic view, HTML(Hyper Text Markup Language) is the language that your browsers uses to display your website.Then CSS(cascading style sheet) is the language that the browser uses to decorate your website.It is what makes the texts, images, pictures and many other website components to look white, green or what ever colour and dimensions the programmer choses.You can think of HTML as the house while CSS is the paint.
Please note that this article is for beginners, newbies and possibly non-programmers. It is a very simplistic overview meant to provide a high level understanding.