Mailbox enabled accounts. Regular users with MAPI mailboxes Active Directory accounts.
Mail-enabled users. Contractors who have an Active Directory logon but no mailbox. (No mailbox in your Exchange Organization.)
Contacts. Suppliers, customers, people with email address outside your organization. No Active Directory account. Thankfully, contacts have a different symbol in the GAL.
Distribution groups. These can be Global or Universal Groups, but they are designed for email rather than security. These are sometimes referred to as DLs – Distribution lists instead of distribution groups. Pay attention to detail and examine the Members and MemberOf tabs.
Query-based distribution groups. Well worth setting up. Again, note the different symbol from other groups. Incidentally, I wish Microsoft would use different colors for different scopes of group. Say, Red for Universal and Green for Local Groups.
Mail-enabled groups. Security groups that have mailboxes. Guy says that unless you have a good reason, favour the classic Distribution group and avoid Mail Enabled Security Groups.
Public Folders. Mail-enabled public folders if your users need an easy way to post.