Is xxx.domain.com and www.domain.com the same?

[r18net]

can be. I've used it before for websites (such as DKP hosting - think WoW) and with a .htaccess file could load different settings based on the subdomain called. Essentially using one install of a piece of software but loading different configs from subdomains.

As for a good thing in the sense of our type of content... Probably not, but then I could be wrong. Your content 'could' be classed as duplicate, and as such could suffer a penalty in SEs (or even flagged as copied).

But it 'could' also be used (in conjuction with htaccess or smart coding) to deliver different content. For example, I am playing with the idea of seperating out my TGP/MGP by using wildcard DNS.
babes.mysite.com will open www.mysite.com/babes.html (or PHP) but to the end user they will still be on babes.mysite.com

Lots of stuff you can use with this, but just be aware if you are on shared or reseller hosting (and your host is an ass) you will find they will be reluctant to change the one line of code in the httpd.conf file for your domain.
VPS and Dedicated ppl, the changes are made in httpd.conf (might want to do a search for yourself where your copy is), and effectively do the following:

Find mysite.com in httpd.conf

Should look something like this:
<virtualhost 1.2.3.4>
Serveralias mysite.com
***** BLAH BLAH BLAH *****
</virtualhost>

And change the ServerAlias line to read this:
ServerAlias *.mysite.com mysite.com

Save the changes, exit the editor, then
service httpd restart

You should now be able to use 1.mysite.com, 2.mysite.com, etc. etc.

Oh and getting the subdomain when someone hits your site in PHP would look something like this:

<?php
$serverhost = explode('.',$_SERVER["HTTP_HOST"]);
$sub = $serverhost[0];
if ($sub = "www") {
$sub = "";
}
?>

There is your lengthy, and comprehensive answer for this sometimes awesome apache modification.