The Address Book is a collection of address books or address lists. You can use the Address Book to look up and select names, e-mail addresses, and distribution lists when you address messages. When you type a name in the To, Cc, or Bcc box of an e-mail message, Microsoft Outlook automatically checks to see if the name you typed matches a name in the Address Book. If there is a match, the name is resolved — the Display Name and e-mail address are filled in — allowing you to send the message. If there is no match, the Check Names dialog box prompts you for more information or you can create a contact by clicking New Contact. If more than one name contains the letters you typed, you can select a name from the list.
In addition to using the Address Book to address messages, you can look up names and other information, such as office locations and telephone numbers, by typing the name in the “Find a contact” box on the Standard toolbar.
When you first open the Address Book, the default address book (usually your Global Address List) is displayed. You can change the default address book and also set other Address Book preferences, such as which address book to check first when sending a message and the location to store personal addresses.
For more information: (http://office.microsoft.com/en-us/outlook-help/about-the-address-book-HP005242118.aspx)