Domain can be selected for domain level settings and email account defaults.
Accounts are individual email addresses.
Aliases are pseudo addresses, they do not actually exist as an email account and are instead delivered to another existing Email Account in the same domain.
Autoresponders can either be an out of office style vacation messages or a generic autoresponder that sends an email in response to all incoming messages.
Forwards are for forwading existing email addresses to another email address, either within the same domain or to an entirely different domain.