Ok. I am new to this whole thing and am softof confused on a couple of things. I register for a domain name like THISISMYDOMAIN.com. So then when i get my site set up what are sub domains for? I dont know what they are and how they are handled. I see different providers saying "5 free sub domains" or "unlimited sub domains". Please help. Thanks

Subdomains are usually used to provide a distinctive address it an account or a secodary section of a site.

For example if you have MYDOMAIN.COM. If you provide your friend with an account and some space, to keep it seperate you might create a subdomain for them at YOURFRIEND.MYDOMAIN.COM.

Alternatively, you might have web sites on more than one topic and use subdomain names to differenciate between them. FOr example:

TOYTRAINS.MYDOMAIN.COM for a site about toy trains
XBOX.MYCOMAIN.COM for a site about Xbox

These are pretty basic but you get the idea.

And what are the advantages of subdomains you may be asking yourself? Well depending on your provider a subdomain may be able to greatly shorten a url. Such as if you have forums stored at:
http://www.yourdomain.com/scripts/php/forums/netforums/
you could shorten it to:
http://forums.yourdomain.com/

However there are a lot of providers that don't offer pointing to specified directories as part of their plans so basically you're stuck with corresponding directory & subdomain names. For example if you wanted to use forums.yourdomain.com on such a host, the subdomain would have to point to http://www.yourdomain.com/forums and then you'd have to have the files in that directory.

There is still a minor advantage even when it's just corresponding which doesn't really shorten the url much. That advantage is capitalization. While directories are case sensative, the subdomain itself is not. http://Forums.yourdomain.com, http://FoRuMs.yourdomain.com, http://forumS.yourdomain.com, etc... all go to http://www.yourdomain.com/forums/. Whereas http://www.yourdomain.com/FoRuMs/ isn't the same as yourdomain.com/forums/