HTML Stands for "Hypertext Markup Language" HTML is the language used to create web pages. "Hypertext" refers to the hyperlinks that an HTML page may contain.

HTML stands for Hyper Text Markup Language, but you already know that and that wont help. I'll keep it simple:

It is a Language used to write web pages, what ever you see on any website, it has been written in HTML. It is used to make a basic structure of any website, like Ashwin Sharan said it acts as the skeleton for any web page. The images you see, the paragraphs, headings, columns, whatever, it is all written in HTML. Learning HTML is the first step towards web designing. 

After HTML comes CSS which out which HTML looks very ugly, hence CSS are the clothes of that skeleton you made using HTML. It is responsible for the design and colors and shape....