The easy answer is that html is the language of web pages.In Depth:
HTML (Hypertext Markup Language) is a programming language used on the WWW (World Wide Web) in order to create web pages. Web browsers then interpret the HTML and display it so that users are able to view it. HTML is actually a very simple language once the basic concepts are understood. Just remember HTML is a client-side language which means the data is interpreted locally on your computer, not from the server.
W3Schools HTML Tutorial
They define how to display HTML elements.In Depth:
CSS (Cascading Style Sheets) define how to display HTML elements. They are normally stored in Style Sheets (.css), they were added to HTML 4.0 to solve a problem. CSS has been a breakthrough in Web design because it allows developers to control the style and layout of multiple Web pages all at once!
W3Schools CSS Tutorial