What is the difference between mail user and mail contact?

Mail user: This is an Active Directory user that represents e-mail address outside your Exchange organization. Each mail user has an external e-mail address to which all messages sent to the mail user are routed.

Mail contact: This is an Active Directory contact that contains e-mail address information about people or organizations that exist outside your Exchange organization. Each mail contact has an external e-mail address. All messages sent to the mail contact are routed to this external e-mail address.