HTML is the HyperText Markup Language. As indicated by its name, it is
a language (in other words it consists of keywords and a grammar for
using them) for marking up text that is hyper! HTML is an extension to
the fairly commonly used Standardised Generalized Markup Language,
SGML 
The pages in the World Wide Web are held in HTML format, and delivered from WWW servers to clients in this form, albeit wrapped in MIME and conveyed by HTTP, of which more below.
Marking up is an ancient skill developed in the Dark Ages of publishing by guilds of printers, keen on presenting the written word in a pleasant and effective way on the printed page. Typically, in recent years, the skill has diminished with the advent of WYSIWYG (What You See Is What You Get, so called whizzy wig) word processing packages and desktop publishing systems. This need not daunt you, since you do not have to author or prepare material for the World Wide Web in HTML directly, unless you really want to. Typically, an author will write material using whatever word processor they are used to, and then use a filter to translate the output into HTML. We will discuss some of the various filters that are available in later chapters.