Can I have http://whatever.yourdomain.com (subdomains)?
Sure! These are called subdomains. Depending on the plan you choose, you may have a number of sub-domains that you can add to you domain account in your control panel. . If you want you may have an additional FTP login username and password for sub-domain as well. You may also have an additional email account with a separate POP account also. Every additional domain you add to your account will also create a subdomain, for example mynewdomain.com will also be available as mynewdomain.yourdomain.com so you can immediately preview the domain even before the nameserver changes from your registrar take effect. This can all be setup using your control panel and at your domain registrar(where you have your domain registered).