The Web is made out of HTML.
HTML, or HyperText Markup Language is a markup language that is used for making webpages such as this one you're viewing right now. HTML is used for putting in written text, links, photos, and sound and video. HTML is used to mark and describe each of these kinds of content so the web browser can show them correctly.
A.N. Lucas's Guide to HTML https://anlucas.neocities.org/LearnHtml.html
Useful starting point for any new comers to HTML.
HTML5 Tutorial https://www.w3schools.com/html/
Very handy tutorial for learning HTML5, this is the best place to start if you know nothing about HTML.
Neocities HTML Tutorial
I. The comic at the top is from XKCD https://www.xkcd.com/1144/ [Internet Archive, archive.is]
http://www.explainxkcd.com/wiki/index.php/1144:_Tags [Internet Archive, archive.is]