HTML is stand for Hyper Text Markup Language.
HTML is simply a language used to write a basic structure of web pages. The everything you see on websites are written in this language. HTML is the first step to learn web designing. It contains markup tags which are describe and classify the elements in the web pages. The paragraphs, headings and columns are you see on the web page, written in the HTML.
The HTML gives you a skeleton of the web page. This is the text file used on Notepad or Notepad++. An HTML file must contains the .html extension.