HTML or HyperText Markup Language is a standardized coding language that provides the content and structure for a webpage. HTML doesn’t specifically tell a computer to do anything, therefore it is not considered a programming language.
HTML is made up of elements which are the building blocks of the webpages. These elements are represented by <> tags, such as paragraphs <p>, divs <div>, headings <h1><h2><h3><h4><h5><h6>, etc. and each element tag must have a closing tag formatted as </p>, </div>, </h1>, etc.
HTML5 is the latest variation of the HTML language format. HTML5 declared some new structural elements such as <section>, <article>, <nav>, <footer> and <aside>, as well as elements for multimedia <audio> and <video>. It also defined new semantic elements such as <mark>, <figure>, <figcaption>, <data>, <time>, <output>, <progress>, <meter> and <main>.
The best place for more information on HTML is from the Mozilla developer website.