Let me put this in very simple terms.
There are two things mail clients and mail servers. Mail servers receive mails and distribute them in various accounts kept at the servers. Now whoever knows the password for a mail account can use a software installed in his machine (called mail client e.g. Outlook Express, Outlook, Incredimail, Ximian Evolution, KMail, Bat, Mozilla Mail, Courier, etc.) and retireve those mails from the server.
I hope I've been able to put it in simple enough terms to understand
