HTML defines data types for the contents of html element, e.g. script data and stylesheet data, and some types for attribute values, including IDs, names, URIs, numbers, units of length, languages, media descriptors, colors, character encodings, dates and times, and so on. For more details review HTML Data Types on wikipedia.
The first document on HTML was "HTML Tags", mentioned on the Internet by Tim Berners-Lee in late 1991. The document describes eighteen html elements that initate the initial and simple design of HTML.
Html document contains tags usually in pairs. HTML elements are generated from tags when parsed by the browser. For more details review Elements vs Tags on wikipedia.
HTML pages are delivered from server to client as documents. The browser then parse this document into document object model which is understood by the browser. Browser then display the HTML DOM into a user friendly view. For more details please follow Document vs. DOM
HTML(Hyper Text Markup Language) is a markup language for describing web documents. Consist of markup tags which describes different document contents.