HTML (the Hypertext Markup Language) and CSS (Cascading Style Sheets) are two of the core technologies for building Web pages. HTML provides the structure of the page, CSS the (visual and aural) layout, for a variety of devices.
HTML is the publishing language of the web, developed by British scientist Tim Berners-Lee in 1990, it is the "hidden" code that helps us communicate with others on the World Wide Web (WWW).
Utilizing HTML tags you can create a web page that can be displayed in any browser across the web. These tags tell the browser how to display the text or graphics in the document.
CSS is a language used alongside HTML to separate a webpage's content (the HTML part) from the design and formatting (the CSS part). It defines fonts, colours, margins, lines, height, width, backgrounds, images, positions and other visual aspects of a webpage, something which HTML was not originally designed to do. Simply put, CSS decides how your HTML is displayed.