Exchange Server 2003 has introduced a new type of distribution list: the query-based DL. These are, as implied by name, distribution lists that are built on the fly based on a query; specifically an LDAP query against Active Directory.
Anyone who has used a distribution list knows how useful they can be. It is much easier to send email to a single email address than to tens or hundreds or even thousands of addresses. Exchange allows you to mail-enable any group object in Active Directory. Then, when someone sends email to that group, every mail-enabled or mailbox-enabled user in the group will receive a copy of the email.