HTML is a language for describing web pages.
HTML stands for Hyper Text Markup Language
HTML is not a programming language, it is a markup language
A markup language is a set of markup tags
HTML uses markup tags to describe web pages 
HTML Tags

HTML markup tags are usually called HTML tags
HTML tags are keywords surrounded by angle brackets like
HTML tags normally come in pairs like and
The first tag in a pair is the start tag, the second tag is the end tag
Start and end tags are also called opening tags and closing tags
HTML Documents = Web Pages
HTML documents describe web pages
HTML documents contain HTML tags and plain text
HTML documents are also called web pages

The purpose of a web browser (like Internet Explorer or Firefox) is to read HTML documents and display them as web pages. The browser does not display the HTML tags, but uses the tags to interpret the content of the page:

My First Heading

My first paragraph

Example Explained
The text between and describes the web page
The text between and is the visible page content
The text between

and

is displayed as a heading
The text between

and

is displayed as a paragraph