A query-based distribution group provides the same 
functionality as a standard distribution group. 

However, instead of specifying static user memberships, you 
can use an LDAP query (for example, "All full-time 
employees in my company") to dynamically build membership 
in a query-based distribution group. 

This reduces administrative costs because of the dynamic 
nature of the distribution group. However, query-based 
distribution groups have a higher performance cost for 
queries whose outcome produces many results. 

This cost is in terms of server resources, such as high CPU 
usage and increased memory usage. This increased usage 
occurs because every time an e-mail message is sent to a 
query-based distribution group, an LDAP query is executed 
against Active Directory to determine its membership.