After you click "Create," the control panel will show you the login information for the FTP account you just created. You may notice that your FTP username has @yourdomain.com after it. This is because your domain is using a shared IP address. If your web site has its own dedicated (static) IP address, your FTP username will not have @yourdomain.com after it.
If you have a dedicated IP address and you create additional domains on that account, these additional domains will have FTP logins of username@thatdomain.com.