Notepad is a basic text editor that you can use to create simple documents. The most common use for Notepad is to view or edit text (.txt) files . Text files are files that contain simply character data, e.g. letters and numbers. So, text files have no pictures, and even no colored text or text of different sizes.

HTML files (web pages) are text files. HTML pages can have pictures and colored text and other features when they are displayed in a browser. But when viewed in a text editor, HTML files are just text with markup. The markup is more text that describes how text is to be displayed. If you right-click on this web page (the one you are looking at now) and choose view source, you can see the HTML text file for the web page that  you are reading now. This HTML text file has markup such as the following:

<strong>markup</strong>

that instructs the browser how it should display the page that you are viewing. The text in angle brackets ("strong") is called "markup".

Many people use Notepad or another text editor to write HTML, although there are also specialized programs for this.