HTML is a standard markup language used to create web pages and interfaces. Most of the websites you visit are sets of HTML files, converted to ‘graphic mode’ by your browser.
Basically, HTML file is a list of rules which defines layout and structure of a web document and tells the browser how to display it. In other words, it’s the art of turning programming code into modern responsive design.
HTML distinguishes between ‘content’ (text, images, video etc.) and ‘form’ (instructions on how to display elements). While the file contains both, you see only the interesting part, leaving boring code lines behind the screen.