HTML or HyperText Markup Language is the main markup language for creating web pages and other information that can be displayed in a web browser. It is the job of a web browser to correctly interpret the HTML and display a web page.
The look, feel and layout of the web page is mainly controlled by a different language - CSS - although HTML also plays some role in this.
HTML is written in the form of HTML elements consisting of tags enclosed in angle brackets (< >), within the web page content. HTML tags most commonly come in pairs of opening and closing tags such as <h1> and </h1>. Between them is the content that they control.
Some tags actually contain the content which they control, and so do not have a closing tag. An example is <img>.
These tags represent the "mark up" of the content and are used to control things such as headings, paragraphs, lists, links, quotes, images forms and so on.