Exchange Server 2007 we have four different mailbox types:
User mailbox: This is a traditional mailbox.
Resource mailbox: This is a mailbox specifically assigned to Meeting Rooms. Its associated user account will be disabled in Active Directory.
Equipment mailbox: This is a mailbox specific to resources, (i.e. TV, Projector and so on). As with a Resource mailbox, this kind of mailbox will disable a user in Active Directory.
Linked Mailbox: This kind of mailbox will be used in environments with multiple forests.