POP3 Accounts are actual mailboxes; i.e., user@yourdomain.com.
ALIASES are virtual mailboxes that forward mail to a POP3 account.
A smaller company wishing to appear larger may have aliases set up for orders@yourdomain.com, customerservice@yourdomain.com, and info@yourdomain.com. Mail sent to these aliases could all be forwarded to user@yourdomain.com.