There are three types of integrations with Microsoft Office:
SharePoint Protocol Integration: This is the official integration between modern versions of Alfresco and modern versions of Microsoft Office.
Microsoft Office Add-in: This used to be an official integration between versions 3.3 of Alfresco and below and version Microsoft Office 2003 and earlier. Though it is no longer part of the product, the code is available as part of a Google Code project: http://code.google.com/p/alfresco-ms-office-plugin
There are some third-party integrations offered by Alfresco Partners and community members.