In Microsoft Outlook 2007, the "autocomplete cache" is a list of commonly used email addresses that Outlook stores in a drop-down list which appears whenever you begin typing an email address. It allows you to "autocomplete" an address by simply selecting it from the drop-down list and pressing Enter or Tab. The email addresses stored in this list however, are different and separate from those stored in Outlook's Contacts folder.
Unlike other email programs, Outlook does not provide an option to automatically store recipient email addresses in its Contacts folder. To add an address to the Contacts folder, you must right click an address and select Add to Outlook Contacts. (You can also add contacts via the New Contact option on the Contacts page)