Create a Contact Group (formerly Distribution List)
A Distribution List (or Contact Group) is a collection of contacts. A list can be created that contains the addresses of all members needed. A message sent to this list goes to all recipients listed in it. Recipients see their own names and the names of all other recipients on the "To" line of the message instead of seeing the name of the distribution list.
For more information see http://office.microsoft.com/en-us/outlook-help/create-and-edit-a-contact-group-formerly-distribution-lists-HA010354963.aspx.