Basically, a website is a collection of documents called webpages (or pages for short) that contain information, like images, words, videos, or sound. They can also contain apps, like messengers, games, or work tools. Pages on a website are linked together by hyperlinks, which are spots on a page (usually text or images) that, when clicked, take the user to different location. This can be another page, or another website altogether.
Webpages are written in a programming language called HyperText Markup Language (HTML). HTML tells web browsers (the programs used to surf the Internet, such as Internet Explorer, Firefox, Opera, Safari, or Google Chrome) what stuff to display and how to format it.
Here's a cool video that gives a quick introduction to web programming languages: