Preformatted text is just that - preformatted; the line breaks are determined by hard returns (hitting the return key). If you don't indicate line breaks, you won't get any. The preformatted text will just trail off the right-hand side of the page. Tele type, however, is treated like normal text. It can accommodate formatting tags, like <p> and <br>, and will automatically wrap, in accordance with the width of the browser window.

The HTML for teletype would look something like this:

<tt>Mom and Dad,

<p>Work is good. Thanks for the shirts. I'm not sure how to tell you this, but, well, I'm quitting my job to pursue professional broomball. This Web stuff wasn't really my thing.

<p>Could you send a few extra dollars my way? I have an appointment to get my shoulder reset, and they make you pay up front.

<p>Your loving son,<br>
Luke </tt>

And would look like this:

Mom and Dad,

Work is good. Thanks for the shirts. I'm not sure how to tell you this, but, well, I'm quitting my job to pursue professional broomball. This Web stuff wasn't really my thing.

Could you send a few extra dollars my way? I have an appointment to get my shoulder reset, and they make you pay up front.

Your loving son,
Luke

Since teletype is treated like normal text, it can be modified by other font tags or integrated into a line of normal text without being pushed onto a new line.

So if your HTML looked like this:

<p>My programs used to say <tt>"Hello, world!"</tt> Now they say <tt>"Bite me!"</tt>

My programs used to say "Hello, world!" Now they say "Bite me!"