Hi & Welcome
Generally the bandwidth & disk space that comes with your account is the total amount that you get - so yes, that would be for all of the sites you develop on that account. Of course, most offer more than enough for any webmaster. By the time you go over those limits... you should be making enough money to open new accounts or upgrade your account without a second thought anyway.
A domain name looks like this:
http://www.domainname.com
A subdomain looks like this:
http://subdomain.domainname.com
I hope that helps! Let us know if you have more questions