HTML stands for HyperText Markup Language. Pages on the World Wide Web are marked-up in HTML. Similar to CSV files, HTML files are stored in plain text format and can be edited using plain text editors like Notepad. On the server, a HTML file usually has a .html extension, although this is not strictly required, for instance .htm extensions are also common (this comes from the days when all filename extensions were three letters long).
HTML is the core front-end coding language of the web. It’s a markup language that contains other front-end coding languages such as CSS (a styling language) and JavaScript (a “high-level programming” / scripting language that runs within the browser). Tables are coded in HTML as are lists, headings, form inputs, etc. HTML is also used to embed richer media formats such as images, videos and on-page Flash or Java-coded apps.
Relation to other markup languages
HTML itself came from SGML (Standard Generalised Markup Language), indeed it is a version of SGML, using the standard SGML syntax. XML (Extensible Markup Language) is also a version of SGML, and when HTML comes together with XML we have XHTML – an XML compatible version of HTML (or an HTML compatible version of XML).
Invention of HTML
HTML was invented by Tim Berners-Lee, founder of the World Wide Web Consortium (W3C) which still formalises the standards for HTML and related technologies like CSS to this day, with the help of its member organisations like Google and Microsoft.
Who should learn HTML
HTML is usually the first coding skill a web developer would learn, followed by CSS, and then JavaScript (for advanced client-side (browser side) functionality) and a server-side language like PHP, ASP, etc.
HTML is also a very useful skill for copywriters and editors to learn, as it helps them to format their articles for the web without relying entirely on the visual editing interface of a content management system (CMS) such as WordPress (and it often equips them to easily manually fix things that may have been messed up and hard to fix within a visual CMS).