An HTML document is segmented by HTML Tags, each segment is called an element. This segmentation defines the semantic structure of the document. Web browser uses this informaton, plus layout instructions to render the page. You can read more about HTML elements from here.
Here is the source of a simple page.
<html>
<head><title>Welcome</title></head>
<body><h1>Welcome</h1><p>Hello, I am a first year graduate student of<a href="http://www.ma.utexas.edu">Department of Mathematics</a>, the<a href="http://www.utexas.edu/">University of Texas at Austin</a>.</p><p>I teach<a href="408C.html">M408C: Differential Calculus</a>this semester. You can find the first day handout from<a href="408c_handout.pdf">here</a>.</p></body>
</html>
HTML tags comes in pairs, an opening tag and a closing tag, like <p> and </p>, <body> and </body>, <a> and </a> etc. HTML tags can have optional attributes, like <a href="408c_handout.pdf">here</a>, the href attribute specifies URI of the link.
As you can see, the sample document is enclosed in a pair of html tags, signaling that the document is an HTML document. The document contains two sections, a HEADER section (enclosed by the head tags) where we include some meta info about the document, and a BODY section (enclosed by the body tags). The BODY section contains a h1 heading and two paragraphs.
If you are interested in learning more about HTML, the W3C schools site is highly recommended. A very useful feature on this site is the "Try-It-Yourself" section, it allows you to try and test your code interactively.
For casual publishing, however, you can avoid the burden of learning HTML by using a WYSIWYG editor.