internal url                                 zone
public url for zone
http://litwareportal:55092             Default
http://litwareportal:55092
http://litwareportal:40033             Extranet
http://litwareportal:40033

I created a site collection for this web application, so I can can
open a browser and type in http://litwareportal:55092/sites/intranet/Pages/Default.aspx
and see the "intranet" site collection. OK.

As it shows above, I extended this into the Extranet zone, but so what
does that mean?
Can someone explain how I use this? I mean, I can't type in
http://litwareportal:4033/sites/intranet/Pages/Default.aspx. I get
page cannot be displayed.

The purpose is to make the http://litwareportal:55092/sites/intranet/Pages/Default.aspx
accessible to a "different" set of users using some other url, but
what is that url?

here are a couple of links [1, 2] that help explain what AAM does.  Suffice
to say that AAM allows you to have more than 1 (up to 5) different URLs that
point to the same content.  That way all you have to do is make one
modification (to your default URL/content) and it will be displayed
immediately when you access the content through different URLs.

Keep in mind that the AAM is only for root level URLs.  If you try to narrow
things down to specific sites within your MOSS/WSS environment AAM will balk
and not work.

Example:

Default Zone:  http://internal.contoso.com
Intranet Zone: http://internal
Extranet Zone: https://external.contoso.com
Internet Zone: http://internet.contoso.com
Custom Zone:  http://admin.contos.com

All of these URLs would point to the same default content but would be
access through different forms of authentication.  You just have to keep in
mind that you have to have DNS entries for each of the URLs so that your
security solution (ISA, SiteMinder, etc) knows which port and URL someone is
attempting go to.  MOSS takes it over from there and displays the correct
URL in the address bar while the content is still the same across your farm.

HTH

Thaddaeus.

[1]
http://blogs.syrinx.com/blogs/sharepoint/archive/2008/04/15/moss-2007...
[2]
http://blogs.msdn.com/sharepoint/archive/2007/03/06/what-every-sharep...