Your browser receives HTML, CSS and other files from the web server where the site is hosted. The browser interpets the data and creates a page you can access. The rendering also places video, audio and interactive elements on the page.
Hypertext Markup Language, referred to as HTML, is the fundamental language of the web. You can think of it as the 'skeleton' or the bones of a website. It declares the structure of a page and contains the text. This very page you're reading now is written in HTML, as are all webpages you've read.
CSS, or Cascading Style Sheets, is a presentation language. It's used to style websites. It also assists web developers and designers in layout and manipulation of elements on a page. If HTML provides a skeleton, CSS is the skin.