Microsoft TechNet article User, Mailbox, and Contact Administration, begins with:
Topics on this Page Understanding Users and Contacts Understanding the Basics of E-Mail Routing Working with Active Directory Users And Computers Managing User Accounts and Mail Features Managing Mailboxes Managing ContactsOne of your primary tasks as a Microsoft Exchange administrator is to manage user accounts, mailboxes, and contacts. User accounts enable individual users to log on to the network and access network resources. User accounts are the only Active Directory directory service objects that can have Exchange mailboxes associated with them. Contacts, on the other hand, are people that you or others in your organization want to get in touch with. Contacts can have street addresses, phone numbers, fax numbers, and e-mail addresses associated with them. Unlike user accounts, contacts don't have network logon privileges.