Web pages (in their most formal divisions) can be broken into the following sections:
<html> <head> <title></title> </head> <body> </body> <address> </address> </html>
The header section tells the web browser things about the web page, it can include all kinds of things, but one of the things that it should always contain is the title. Whatever text is within the title tags will generally be displayed at the very top of the web browser's window (ex. Take a look at the window name when the web browser is looking at this page) and will be used as the title of the page in the web browser's history list.
The beginning body tag tells the browser things about the body of the web page itself. Again, all kinds of things can be put here. This is where you can change the colors of the background, text, links, and visited links as well as use images for the background of your page.
Within the body tags is where the lion's share of your HTML will go, and is very similar to the body of a written letter as far as its function goes.
The address tags generally enclose the contact information for the webmaster or author of the page.
Ultimately all of these sections are optional, but when you break up your HTML into these sections it helps the browser that is reading your HTML understand what is going on.