The Global Address List (GAL) is a list of objects that have Exchange attributes that match a pre-defined query. Typically it lists all the Exchange users/mailboxes, distribution lists and contacts in an organisation.
Understanding Address Lists]
[http://technet.microsoft.com/en-us/library/bb232101(EXCHG.80).aspx Managing Global Address Lists]
How to make sure that Outlook clients access the correct global address list if there is more than one global address list
How to create an offline address list that contains a filtered copy of the global address list
How to work with the Exchange Recipient Update Service