HTML (Hyper Text Markup Language) is a markup for describing the way a web page will look. HTML is written in the form of tags; the first of a pair being a word surrounded by angle brackets, the second of a pair being the word in angle brackets, with a forward slash before the word, but after the first angle bracket.
Sample Web Page
<html> <head> <title> Hello World </title> </head> <body> <p> Hello World </p> </body> </html>
CSS (Cascading Style Sheets) can be used to set what things will look like for multiple HTML pages, so, when you want to change the look of your website, instead of going through every one of your HTML pages, you can change the CSS, and the HTML pages will change as well.