Almost -- subdomains are usually foo.domain.com, but you can have any manner of subdomains and redirect them however you like (your DNS record has to be setup properly).
A domain has the following components
host = a name of a resource on a domain. Usually this is www indicating a web server, but it doesn't have to be. For example, images.domain.com, ftp.domain.com, mail.domain.com -- or for example, rotator.juggler.inetinteractive.com (the adjuggler server that inet uses).
sld = second level domain -- this is what you purchase when you are buying a domain. Anything under this (to the left of the . ) is called a subdomain.
tld = top level domain (.com, .net, etc).
Hope this helps and I didn't confuse you.
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