HTML (Hypertext Mark-up Language) is the language used to create web page documents. There are a few versions of HTML in use today: HTML 4.01 is the most firmly established and the newer, more robust HTML5 is gaining steam and browser support. Both versions have a stricter implementation called XHTML (extensible HTML), which is essentially the same language with much stricter syntax rules.
HTML is not a programming language; it is a mark-up language, which means it is a system for identifying and describing the various components of a document such as headings, paragraphs, and lists.
The mark-up indicates the document’s underlying structure (you can think of it as a detailed, machine-readable outline). You don’t need programming skills—only patience and common sense—to write HTML.
The best way to learn HTML is to write out some pages by hand.