When Outlook client is configured, you may see the following Address Books in the list:

For Exchange 2010/2013/2016 mailbox:

Contacts it is a list custom Address Book. It will display the list of contacts that were manually added by the user.

Global Address List (GAL) is a contact folder that is stored on the Exchange server which contains information about all email users, distribution groups, and Exchange resources. Read the Knowledge Base article on What Is The Global Address List? for more information.

Offline Address Book (OAB) will only be available when Outlook is used in cached mode. Read the Knowledge Base article on Offline Address Book (OAB) for more information.

All Rooms Address List contains the list of all Resource Mailboxes (i.e., rooms and equipment) on your account.

Company Address List This Address Book is the same as GAL.

For Exchange 2007 mailbox:

Offline Address Book (OAB)  will only be available when Outlook is used in cached mode. Read the Knowledge Base article on Offline Address Book (OAB) for more information.

Global Address List (GAL)  is a contact folder that is stored on the Exchange server which contains information about all email users, distribution groups, and Exchange resources. Read the Knowledge Base article on What Is The Global Address List? for more information.